Unanswered Questions of the Cuban Missile Crisis
September 26, 2012
Graham Allison and Philip Zelikow
Why did Khrushchev risk this enormous, secret deployment?
Why were the ballistic missiles discovered? On October 14, Pliyev suspected the missiles had been discovered; on October 18, Gribkov was informed of this suspicion. Why did the Soviets in Cuba not transmit this back to Moscow?
Did the strategic nuclear balance of the time significantly impact the choices Kennedy and Khrushchev made during the crisis?
Withdrawal of Missiles: how significant was the secret concession on Turkish missiles on Khrushchev’s decision? Other influence?
How serious was the risk of war? Would Kennedy have ordered an airstrike on Monday (October 29) if Khrushchev had not met his 24-hour ultimatum?
How significant was this crisis in the course of the Cold War and world history?
What were the central lessons Khrushchev and other Soviet leaders took away from crisis? Other world leaders besides the US and Soviet Union?
How did experience and outcome change Khrushchev strategy toward Berlin after--where he essentially accepted status quo?
How consciously did Khrushchev and other leaders, including the military, know that this experience lead to better understanding of what JFK called "precarious rules of the status quo' according to which both US an SU refrained from surprises or provocations that could lead to confrontations that risked nuclear war?
According to Dobrynin’s memoirs: “Soviet leadership could not forget a blow to its prestige bordering on humiliation when it was forced to admit its weakness before the whole world and withdraw its missiles from Cuba. Our military establishment used this experience to secure for itself a new large-scale program of nuclear arms development.”
When was this reaction translated into decisions by the general staff and the Soviet government to significantly increase Soviet nuclear forces? When did new purchases of strategic nuclear forces come into the operational arsenal?
These questions/comments are framed from the perspective of a historian of the Joint Chiefs of Staff, who wrote The Joint Chiefs of Staff and National Policy: Volume VIII, 1961-1964 that included a chapter on the Missile Crisis:
1. Underlying the management of the crisis was a mutual lack of confidence between President Kennedy and the Joint Chiefs of Staff, dating back to the Bay of Pigs. After April 1961, the need to restore U.S. credibility became a running theme in JCS recommendations. The Chiefs’ concern was encapsulated in a memo of 26 October 1962: “The longer we talk, the more diffuse become the inevitable arguments, the weaker becomes whatever may be the final agreement. And when this happens, as it has in the past, we will have lent credence to the impression that we may be a strong country but we are a country unwilling to use its strength. We have the strategic advantage in our general war capabilities; we have the tactical advantages of moral rightness, of boldness, of strength, of initiative, and control of this situation. This is no time to run scared.” They believed that we held all the high cards, and would win the game no matter how it was played. But President Kennedy took military advice at a rather steep discount. We could consider whether more weight should have been given to JCS views, and how an appreciation of U.S. military superiority might have been leveraged during the days of open confrontation/negotiation.
2. Fear of Soviet retaliation against Berlin, possibly the Jupiter sites in Turkey and Italy as well, seems to have been crucial in moving President Kennedy to opt for a blockade. In their meeting with the President on 19 October, the JCS downplayed this danger, perhaps rightly so. Is there hard evidence about Soviet intentions toward Berlin, Cuba, and Turkey?
3. Apparently, our intelligence repeatedly misjudged the operational status of MRBM sites. On 20 October, the day Kennedy decided on a quarantine, the JCS believed that some sites would become operational by 24 October. On 22 October, the day of Kennedy’s speech, the Defense Intelligence Agency reported four sites as being operational, and two more would be soon. On the evening of 27 October, at the peak of the crisis, a DIA briefer told the Chiefs that canvas was off the launchers, missiles were on the launchers, and a reload capability was ready. On the morning of 28 October, just before Khrushchev’s backdown was broadcast, Gen. LeMay said that 29 October would be the last day to strike before the missiles became fully operational; Gen. Wheeler added that according to his information all MRBM sites were operational and could be ready to fire in 2 1/2 to 5 hours if warheads were mated with missiles. But in Operation Anadyr, Gen. Gribkov says that only about half the missiles were ready to be fuelled (itself an 18 hour process), none had been programmed for flight, and warheads remained in separate storage. Evidently, therefore, we had more time for decisions and preparations than we believed. We seem to have been relying entirely on photo interpretation, which in this respect led to inaccurate conclusions. The information provided by Col. Penkovsky about missiles has been much praised, but did it prove irrelevant on this key point?
4. How important was the meeting between Robert Kennedy and Amb. Dobrynin on the evening of 27 October? Had Khrushchev already decided to settle for a no-invasion pledge, so that the offer of a Cuba-Turkey trade was not the deal-maker but simply a welcome bonus? To draw the correct “lessons learned,” we need to be as definitive as possible on this issue. With the wisdom of hindsight, how much give-and-take negotiation was necessary? Had Khrushchev already come to believe that a no-invasion pledge was for him a sufficient face-saver?
5. Concerning the transcripts of discussions by the NSC Executive Committee: I had the impression of constant ad hoc improvisations, of people reacting to daily developments much more than of consistently pursuing a coherent strategy. Almost by default, Secretary McNamara emerged as the dominant figure in ExComm discussions. Possibly this more free-wheeling approach to crisis management appeared to work well because of the unique circumstances in which U.S. military superiority was so pronounced. When this same approach was followed during August-October 1963 over what do about Ngo Dinh Diem, the result was back-and-forth waffling leading to a chaotic outcome. It could be useful to compare Kennedy’s style with that of Nixon and Kissinger, which featured an extreme centralization of decision-making that effectively excluded some principals but in which WSAG/NSC discussions were a good deal more structured because Kissinger usually opened them by listing “realistic” options and describing the pros and cons of each.
I focus on ten aspects, mostly relating to nuclear and intelligence issues, and to some involving the British that may have broader implications.
1. David Ormsby-Gore, the British ambassador in Washington, was summoned for lunch at the White House on Sunday 21 October when he was told of the missiles and the American response. When Ormsby-Gore reported back to Prime Minister Macmillan, he explained that Kennedy said something that was so sensitive it could not be put in a Foreign Office telegram, and he offered to fly back to London to tell Macmillan face to face (the Prime Minister told him to stay put). What was it? I doubt if it related to missiles in Turkey, as Ormsby-Gore explained that the President said the missiles in Turkey were ‘more or less worthless’ and that he would wait to see if a deal on them was possible. My hunch is that it related to Berlin.
2. On 19 November the Director-General of the Joint Intelligence Bureau, Sir Kenneth Strong, briefed Macmillan and Foreign Secretary Home, having been in Washington from 13 to 25 October. Strong had very good connections in Washington (and had successfully served as Eisenhower’s G-2 in the war). He was one of the senior British intelligence officials coincidentally visiting CIA headquarters as the crisis was erupting, and was briefed by Ray Cline about the missiles on 19 October. It is not clear what other access he had during this stay, but he told Macmillan and Home that the Americans, had been, ‘prepared to go it alone either without consulting their allies or irrespective of what their allies said had the Russians reacted against any action in Cuba by moving against Berlin.’ Strong told Macmillan that he thought Washington was prepared for action in Cuba to, ‘escalate into the nuclear.’ Was this the issue Kennedy raised with Ormsby-Gore? Whether or not it was, Strong’s report raises broader issues about Kennedy’s intentions over Berlin.
3. How would Kennedy have reacted in Berlin if Khrushchev had responded to American action in Cuba? Whether Kennedy or Khrushchev would have used force is as speculative as it is necessary in exploring the risk of war. There is also a specific question for the British: did being told that Washington would cross the nuclear threshold over Berlin regardless of the views of NATO allies reinforce Macmillan’s determination to secure a strategic delivery system to maintain the British deterrent (which loomed into focus shortly after the crisis with the cancellation of Skybolt and Macmillan’s successful pursuit of Polaris)?
4. Was Kennedy really prepared to initiate nuclear war over Berlin? If he did say this was it to bolster confidence in his resolve, perhaps comparable to his public declaration on 22 October that any missile launched against any country in the western hemisphere would meet with a full retaliatory response against the USSR?
5. It has been claimed that the espionage of Oleg Penkovsky (involving the field manual for the Soviet MRBMs in Cuba) was crucial to Kennedy’s decision-making from 16-19 October. Further illumination of this issue would be helpful. What we do not know is what Penkovsky told the KGB about his espionage after they arrested him on 22 October, nor what the KGB told Khrushchev about the extent he had exposed Soviet strategic inferiority. Roswell Gilpatric’s October 1961 speech may have signalled that the west no longer believed Khrushchev’s grandiose claims about Soviet strength, but if Khrushchev realised Washington had even greater understanding of Soviet weakness, how might this have affected the trajectory of his thinking that week? There are also questions about the KGB’s activation of the DISTANT warning procedure in November (recounted by Jerrold Schecter and Peter Deriabin and by Peter Hennessy).
6. One specific aspect of Soviet threat analysis concerns UK and NATO forces. In Britain, 59 Thor IRBMs were at less than fifteen minutes readiness (with warheads mated) as were elements of the V-Bomber force (though British bomber preparedness fell well short of American dispersal and airborne alerting). The issue of Soviet perceptions of British (and other NATO) nuclear forces has relevance to the history of the crisis but also to broader debates about the British deterrent.
7. Other questions about Soviet-British relations include whether approaches to the Foreign Office in London reflected a GRU initiative or were a considered political strategy designed to foment divisions with NATO. Some of the questions about Yevgeny Ivanov’s activities in London are comparable to those concerning Aleksandr Feklisov’s activities in Washington.
8. How a nuclear war might have started in October 1962 has been brought into focus by several decades of research and revelation, most notably in Michael Dobb’s book. Whether it would have escalated and how war would have been fought (and terminated) raise huge imponderables. Declassification of war plans and operational information are nevertheless relevant to various issues. There are also specific questions about who in Washington believed an American first strike was possible (Strong told Macmillan and Home that people in Washington believed they could take out the Soviet strategic forces)? Who believed limited Soviet retaliation could be an acceptable cost for an American first strike (or what level or form of retaliation did they consider acceptable)? How was the assured destruction of European targets and cities assessed in these equations? Also, since February 1962 the British Bomber Command had a co-ordinated strike plan with SAC, but they also had a national plan. If Washington withheld attacks on Moscow to preserve the prospect of control and termination of hostilities, would this have been compatible with a British desire to use its more limited and vulnerable forces to maximise damage to the Soviet state?
9. Nuclear command and control remains an area of great significance is assessing risks, in particular of inadvertent nuclear war. The issue of delegated authority loomed large in debates about Soviet short-range ballistic missiles in Cuba as well as in the stories of the Type 641/Foxtrot submarines. Command and control procedures for the Soviet submarines remain opaque. So too does the issue of American assessments involved in ad hoc submarine surfacing procedures. Brugioni explains that American intelligence knew the Type 641/Foxtrots could carry nuclear ordnance. At what point was this considered? And did it feature in the ExComm discussion involving the President and McNamara on the 24th? Another command and control aspect is that while much of the focus has been on tactical systems basic questions nevertheless remain about strategic weapons, including both Soviet ICBMs and strategic bombers, as well Soviet forces targeted on NATO Europe.
10. Soviet (as well as British) SIGINT is an unknown quantity. Some vignettes raise specific and general questions. Fursenko and Naftali explain that SAC’s move to DEFCON-2 was picked up by the GRU. How did the Soviet military interpret this (or the communications they may have intercepted with SAC’s airborne alert forces) and what did they tell Khrushchev? More broadly, how did Soviet threat perceptions develop during the crisis at the military and political levels? When Major Maultsby’s U-2 strayed into Soviet air space, how did the Soviet military assess the threat and how exactly was this presented to Khrushchev? A further question is if people in the Pentagon were exercised that the Soviets might worry this was pre-SIOP reconnaissance and consider launching on warning, what consideration was given (including among senior military officers) to American pre-emption of Soviet ‘pre-emption’?
What would we like to know about the Cuban missile crisis? Or maybe I should say: what would I personally like to know about the missile crisis? I make the distinction not just to suggest that I’m going to talk about a number of issues I happen to be interested in, but which might not be of great interest to other people. I also want to suggest that maybe the answers to some of those questions are already known to other people, at least in part. My problem is that I don’t really know whether they have been dealt with by other people, because I haven’t been able to keep up with the scholarly literature in this area in any systematic way. I do read things relating to the missile crisis from time to time—I just read David Coleman’s new book, for example, because I was asked to write a blurb—but I now only read things related to the missile crisis sporadically.
So let me talk a bit about some of the things that are still question marks in my mind—and a bit also about how one might go about getting to the bottom of these issues.
1. The Question of a Standstill Agreement
Why was this episode a crisis? The Americans had discovered that the USSR was deploying nuclear-tipped, or at least nuclear-capable, missiles in Cuba, and wanted the Soviets to withdraw them. This was clearly a situation they felt they had to deal with, but why the sense of urgency? Why was it felt that the problem had to be dealt with quickly, and through military means if necessary?
Part of the answer is known. It was felt that once the missiles had become armed and operational, an attack would be too risky, and if an attack was no longer possible, the U.S. government would have no way of getting the other side to withdraw the missiles. So the matter had to be settled before things reached that point. But this is not the whole answer, because one could have tried to deal with the problem in two stages. First, there could have been a freeze on construction. Then a permanent agreement could have been worked out. The element of urgency could have been removed from the equation.
And this, as the Bromley Smith ExComm minutes revealed, was what President Kennedy wanted. So the question is: why didn’t things proceed along those lines? Was the standstill idea proposed to the Soviets, and if so, why did they reject it? And if it wasn’t proposed to them, why not? I took a stab at this issue years ago.1 I used some material I had found in the U.N. archives in New York. That material suggested to me that U Thant, the U.N. Secretary-General, essentially fumbled the ball. The Americans had wanted him to propose the standstill idea to the Soviets. He instead proposed it to the Cubans, who were not interested. The Soviets obviously would have been interested in it, since they in the end accepted something far more extreme—a capitulation, not a negotiation.
If valid, this interpretation would be of considerable interest. It would shed some light on the inadvertent war issue, on the general problem of the dynamics of escalation, and also on the question of whether international organizations make for a more peaceful world. But I’m far from sure this business with U Thant is the whole story, or even the heart of the story, and this general issue is certainly worth examining in the light of the massive amount of material relating to the crisis that is now available.
2. Assessing Intent
How do countries draw inferences about each other’s intentions from their observations of what each other is doing? This very important (and, to a certain extent, under-theorized) issue can be studied by looking closely at how each side drew inferences about the other side’s intentions in the case of the missile crisis.
When I did my work on the 1962 crisis, I was struck by a comment General David Burchinal made in an oral history. The U.S. government, as we all know, put the Strategic Air Command on Defcon 2 during the crisis. As Burchinal explained to me when I interviewed him, that meant that fully armed B-52 bombers were flying up to the fail-safe point on the two attack routes (over the pole, plus up through the eastern Mediterranean). But the Soviets were evidently doing nothing equivalent. As Burchinal put it in an oral history interview: “we had a gun to their heads and they didn’t pull a muscle.” That claim, as I recall, was disputed by Scott Sagan, who said the Soviets had indeed put their forces on alert. But what they called an alert—that is, the particular measures they took—seemed fairly minimal—or at least that was the impression I got at the time.
Today presumably a lot more evidence is available. By studying that evidence, we should be able to see what exactly, if anything, the Soviets were doing, and why exactly they took the course of action (or inaction) they did. If they were doing very little, was it because they were afraid that preparations to limit their own vulnerability to an American attack would seem indistinguishable, to the Americans, from preparations to launch a preemptive attack on the United States? And who was calling the shots on the Soviet side? In determining what measures, if any, to take, were the key decisions made by military officers, or were political leaders in charge? We of course already know a bit about such issues, but we could learn a lot more.
But the main questions in this area that interest me relate to the American side. You’d think that U.S. leaders during the crisis would have been deeply interested in knowing what exactly the Soviets were doing, in large part so that they could have some basis for speculating about what Soviet thinking was—that is, for speculating about how far the Soviets were prepared to go in this affair. And yet, judging from the records I saw 27 years ago, U.S. leaders did not seem particularly interested in exploring these issues—that is, both with getting the facts and drawing inferences from them. Could that impression have been mistaken?
The issue is important because it relates to the whole question of the strategic balance. Looking at what the Soviets were actually doing was one way to get some sense for what the Soviet leadership thought the strategic balance actually was. If they did essentially nothing, that must have meant that they felt too weak to even take precautionary measures to lessen their own vulnerability. It might have suggested that they had no stomach for engaging in any sort of “competition in risk-taking,” to use Thomas Schelling’s famous phrase. But did American officials, and especially President Kennedy and his top collaborators, grapple with these issues on the basis of whatever evidence was available? All of this relates to the core issue of how the strategic balance affected the way the crisis ran its course—a very important issue which also need to be revisited in the light of all the evidence that has become available in the last thirty years.
3. The Dynamics of Escalation
In 1990, I published a little piece in Diplomatic History about the missile crisis.2 People were coming to make the argument that the missile crisis was a lot more dangerous than had been thought at the time. It was often argued at that time that the new evidence showed how easily a crisis of this sort could get out of hand. My basic claim was that in fact the opposite conclusion was warranted. The new evidence, I thought, actually showed that the crisis was less dangerous than we had thought.
One of the arguments I made there was that it was a mistake to argue, as some scholars did, that:
the Cubans’ persistence in firing on low-flying American reconnaissance planes was one of the main factors that brought the world “to the brink of war.” This, however, was not nearly as dangerous as we have been led to believe. It is clear from the transcript of the 27 October ExComm meetings that the American response to these attacks was not to order a counterescalation. Cuban antiaircraft fire was instead leading the United States to pull back. As General Maxwell Taylor, the chairman of the Joint Chiefs of Staff, said that day: ‘We‘re approaching the point, I think, Mr. President, where low-level reconnaissance will be entirely impossible.. . . Low-level reconnaissance probably is on its way out.” When one studies these events at this level of detail, one does not detect a process of escalation spiraling out of control. Toughness was met not by countertoughness, but by accommodation.3
Was this a valid argument? Sheldon Stern, in the conclusion of his book Averting the Final Failure: John F. Kennedy and the Secret Cuban Missile Crisis Meetings (2003), commented on this passage as follows:
Some historians, of course, completed their work before all the October 16-28 recordings, the most indispensable recently declassified American resource on the missile crisis, were finally opened by early 1997. Marc Trachtenberg, for example, working with the incomplete and frequently inaccurate 1983 transcript prepared principally by McGeorge Bundy, understandably downplayed the view that the Cuban decision to fire on low-level U.S. reconnaissance planes had nearly pushed the superpowers into war. “It is clear from the transcript of the October 27 ExComm meetings that the American response to these attacks was not to order a counterescalation. Cuban antiaircraft fire was instead leading the United States to pull back.” Trachtenberg cites General Taylor's remark that low-level surveillance will soon become “entirely impossible” and concludes, “When one studies these events at this level of detail, one does not detect a process of escalation spiraling out of control. Toughness was not met by countertoughness, but by accommodation.”
The complete tape, on the contrary, makes clear that Taylor was actually arguing that although low-flying reconnaissance might soon become impossible, medium and high-level mission could still be launched if at least ten SAM sites or “the whole works” were knocked out. McNamara was even more combative, urging the President to continue the low-level surveillance missions with fighter escorts and if the planes were fired on again, to “attack the attackers.” JFK decided instead to wait until Sunday afternoon for a possible breakthrough if U Thant traveled to Cuba. But, if that initiative failed and the Soviets did not respond diplomatically, he recommended making a public statement about the antiaircraft fire and “then go in [on Monday] and take out all the SAM sites.”
The essence of this exchange, as captured on the tape, reveals that Taylor and McNamara were ready to unleash American air power against Cuba without delay. President Kennedy, although similarly pessimistic about diplomatic progress through U Thant or Khrushchev and very uncertain about how the military situation would eventually play out, was nonetheless disposed to put off escalation for perhaps a day or so--just in case. Toughness, the tape demonstrates, was not really met by accommodation, but rather by JFK's fatalistic bur fortuitous decision to at least try to delay countertoughness.4
Was Stern basically right about all this, and was I basically wrong? This is the sort of question one can study in order to get some sense for how the escalatory process works. And looking back, I think the kind of language I used made it seem that tough tactics would never provoke counter-toughness. But that was not what I meant to suggest. I certainly do not think, and did not think when I wrote that article in 1990, that if the SAMs had continued to attack the high-level American reconnaissance aircraft the United States would have done nothing—that the U.S. government would have called off further flights. I was reacting to the argument that a number of people had made to the effect that the Cubans (who controlled the low-level anti-aircraft batteries) could have caused a war on their own, regardless of the Soviet attitude. If the Soviets, who controlled the SAMs, tried to prevent all further overflights, including the high-level ones, that would have been another matter entirely. And the Americans probably would have responded violently if the U.S. planes were shot down by the SAMs.
But my basic assumption here was that the Soviets knew that and had therefore decided to permit the high-level overflights. This was a case of (anticipated) U.S. toughness leading not to countertoughness but to accommodation. It was thus in line with the idea that the risk of things spiraling out of control was not nearly as great as people thought. But it would certainly make sense to try to learn as much as we can about all of these issues, and we can do that by looking closely at how both the Americans and the Soviets dealt with them at the time.
More generally, one could look at the various arguments people have made to the effect that a war could have come even though no one wanted it—arguments to the effect that in nuclear crises the escalatory process is in large measure uncontrollable. One could then systematically examine those arguments in the light of the massive amount of evidence that is now available.5
4. The Cuba/Berlin Link
Some people in 1962 (including many military officers and certain key civilian officials) thought the United States should have gone into Cuba and gotten rid of the Castro regime. The Soviets had lost the test of strength; the Americans, it seemed, could do what they wanted without having to worry too much about what the USSR would do. But the U.S. government, of course, did not go that route, and decided instead that it could live with the Castro government.
Many factors, one could argue, played key roles in shaping that decision, but there is one particular factor that might be worth exploring in a relatively focused way. This is the idea that Kennedy had perhaps reached the conclusion that a Cuba with ties to the USSR might actually be in America’s interest. Just as West Berlin was a source of weakness for America—a kind of Achilles’ heel—and thus a valuable lever for the Soviets, Cuba could be Russia’s Achilles’ heel. Indeed, a Soviet commitment to Cuba could balance out the American commitment to Berlin—the two problems could in a sense cancel each other out. That basic idea would be that f the Soviets moved against Berlin, the Americans could respond by moving against Cuba—a prospect which might discourage the USSR from moving against Berlin in the first place. But that tactic could be used only if the Americans did not overthrow the Castro regime.
Is it conceivable that Kennedy, at least, was looking for some way to stabilize the situation in Europe, and that a relatively moderate policy vis-à-vis Cuba (and the USSR as well) made sense in that context? This was particularly true, given that while the United States still enjoyed an important strategic edge, Kennedy knew that that situation would not last forever. (Indeed, in September 1963 he was told by the Net Evaluation Subcommittee of the NSC, the group whose job it was to study these issues, that America no longer had a meaningful advantage in strategic nuclear terms—that the level of devastation would be so great that preemption was no longer possible.6) Given that America’s window was closing, maybe it made more sense to think about how the United States and the Soviet Union could live with each other in a world of nuclear parity than to take advantage of the fact that America still had the upper hand in strategic terms and could dicate humiliating terms to the Soviets. Or to put it a somewhat different way: as Kennedy often said, it was America’s strategic superiority that had made it possible, in the final analysis, to protect Berlin; it followed that, with U.S. superiority soon to become a thing of the past, America needed another way to safeguard the status quo in Berlin. Creating a more symmetric structure, with a U.S. commitment to Berlin balanced by a Soviet commitment to Cuba, might provide a certain degree of stability in the kind of world that was about to come into existence.
The whole question of the role that strategic considerations of this sort played in U.S. policymaking during the Kennedy period has, to my mind, been understudied, and it would make sense to look systematically at the evidence with these issues in mind. And one can do a lot of that work quite quickly with the technology that is now available. One can, for example, go into the key sources—especially the volume in the State Department’s Foreign Relations of the United States series on the missile crisis, the three volumes in the Miller Center’s Presidential Recordings collection dealing with the “great crises,” and the Digital National Security Archive collection on the Cuban missile crisis (all available online)—and do keyword searches for words like “Berlin” and “nuclear.”7
I do not mean to imply that scholars have totally ignored these issues. I myself referred to the “mutual hostage relationship” between Cuba and Berlin in my 1999 book and cited a couple of documents in that context.8 David Coleman, in his new book, also shows that Kennedy was thinking along those lines. He quotes from the tape of the president’s private briefing of Congressional leaders not long after the crisis, on January 8, 1963. “I think if Berlin gets difficult,” Kennedy said, “we’d always—they have now given—Cuba’s almost the same position Berlin was with us for a decade. Any action they take in Berlin we can take an action in Cuba..”9 Evidence of the sort is of course quite interesting, but we can, and should, go into these issues in much greater depth.