The Navy’s finding was that Berry Cannon died because the canister in his breathing rig held no Baralyme, and that the chief who had certified the rig had let it go down empty. Fifty-five years later eleven of the surviving aquanauts put their names to a statement that the chief had done no such thing, and the engineer who gathered their account argues that Cannon was electrocuted. This piece sets out what is known about the four rigs, what the board of investigation heard, and what has been argued since. It does not settle it. Nothing available can.
The rig
A semi-closed breathing apparatus works like this. A small, steady flow of fresh gas, helium and oxygen mixed for the depth, is metered into a bag on the diver’s back. He breathes from the bag and back into it, through a canister of granular Baralyme that takes up the carbon dioxide he has made; what the flow adds beyond what he uses bubbles away. The diver’s own lungs push the gas round. The Navy’s Mark VIII was a rig of this kind, designed to be fed by an umbilical from the habitat. For use from the transfer capsule, where there was no umbilical, it was modified into the Mark IX, with its own gas bottles and a by-pass the diver pressed when the bag ran short, which the divers found they had to press often. There was no full-face mask that would seal with it, so the men wore an ordinary mask and held a mouthpiece in their teeth, and could speak to nobody.
In May 1969 the trade paper Ocean Industry reported from the hearings that “the Mark IX had not been previously used in training dives deeper than 60 feet, and late modifications, designed to prevent breathing gas cutoff, were not tested at EDU to ascertain any breathing restrictions they might cause under heavy work.” The gas in a hose at 610 feet is eighteen and a half times as dense as at the surface. Richard Blackburn, who dived twice with the rig: “Semi-closed and closed-circuit breathing systems require the lungs to do all the work of recirculating the gas through the apparatus and CO2 absorbent. The least amount of breathing resistance in or out will have a devastating effect on the diver.”
The first dive
After the first dive, on 16 February, the four rigs went up to the diving locker on the Elk River with a note from Blackburn to Senior Chief Torpedoman Paul Wells, who ran the locker, “because Berry and Bob had such a hard time breathing and both had mental problems after their first dive.” The note survives in his after-dive report: “Check that buddy breather slide valve is on when rigs are sent down. Because the second stage + hose between the slide valve and second stage will be squeezed if not left on. I think there is a great restriction in the inhalation hose because of the stiffener in between the two bag fittings. Take a look, that is a small hole to have to suck through at 600 feet.”
Keith Moore, a Navy diver, helped Wells reload the rigs for the second dive. He remembers the locker as mayhem and the two of them as the only people in it who were not distracted, and the strap wrench they needed to open and close the canisters. He carried the four rigs to the capsule himself. He is certain he would have noticed an empty one by its weight. The divers’ own check before a dive, he believes, would have found it too. Of Wells, every aquanaut who has spoken says the same thing. Andy Pruna: “P.A. was the most capable guy, the most meticulous. I’d dive a rig he prepared any day.”
The half hour
When Cannon was brought into the capsule on the second dive, Barth’s, Blackburn’s and Cannon’s rigs were left outside it, and the capsule went up at once. The rigs rode to the surface on the outside of the sphere, dangling about a hundred feet below it, for the half hour and more the ascent took. On an ordinary dive they would have come up inside it. In a semi-closed rig carried up through 600 feet of water the gas in the bag and canister expands eighteen times over and vents; a canister open to the sea can flush. Baralyme residue was found in the exhaust path of the rig whose canister was empty, which Hardy reads as the mark of a canister that had been washed out rather than one that had never been filled. Martin Harrell, an aquanaut of Team 5, recalls a training dive on which the canister of his own rig “emptied its baralyme into the sea,” which he offers as “an alternate explanation for how the CO2 canister of a Mk IX HP-SCUBA could have been emptied.” It was also, Hardy notes, standard practice to empty a canister of wet Baralyme so that it did not corrode the rig, though nobody suggests that was done here.
And nobody could say whose the empty rig had been. The board heard from the project’s communications officer, Lieutenant William Marsh, that the videotape of the dive could not identify the rig Cannon wore; the Long Beach paper reported that the same rig “had a hole in its breathing hose.”
The board
The board of three officers sat in San Diego from 26 February to 12 March 1969, much of it closed. The coroner had found carbon dioxide poisoning, with acute haemorrhage in the lungs, and Time reported “a greatly excessive” quantity of carbon dioxide in Cannon’s blood. Hardy’s objection is that the toxicologist’s analysis of air from the trachea was made many hours after death, and that the findings at post-mortem alone could not fix a cause. Wells’s counsel, Lieutenant Alan Goldberg, put to the board more than once that an alarm system which had sounded when Cannon was in trouble could have put a shock into the water. The board closed without establishing which rig Cannon had worn, and the official finding was carbon dioxide poisoning. Martin Harrell, who was aboard the Elk River monitoring the gas, wrote in 2024 that he had heard from people at the project office whom he trusted that the board’s first finding was that it could identify no cause of death, and that the Chief of Naval Material kept it in session until it found one. That is hearsay, and the board’s report has never been released to test it. Wells was reprimanded and left the Navy. Bond wrote in his journal: “Not one of us believed that Paul Wells would ever load an empty canister into a Mk IX rig.” That summer the Experimental Diving Unit and Duke University put five divers into a refrigerated chamber at the pressure of 600 feet for a fortnight to test the Navy’s equipment in cold water and, as a congressman put it to the House, “to simulate the conditions existing at the time of the death of Aquanaut Berry Cannon.” He added: “The exact cause of his death has never been determined.”
The cable
Hardy’s case rests on the habitat’s electricity. Its power came down the main umbilical at 440 volts, three-phase, from the Elk River. The procedures said: “All electric circuits are ungrounded to eliminate shock hazards. However, since an accidental ground in the system could create a shock hazard, the cases of all portable electric equipment must be grounded to the hull.” The hull, that is, was the ground. There were no breakers on the main cable, only ground-fault alarms, and before the dives they were sounding. The log of Howard Talkington, who ran the ocean technology branch of the Navy’s laboratory in San Diego, records the ship’s engineers isolating circuits one by one and concluding that the fault lay in the capsule or the habitat; with both capsules then on deck, Hardy points out, that left the habitat. Helium leaking from the habitat had got into the cable jackets and burst them in several places, which the trade press reported at the time. After the accident Larry Hallanger, an aquanaut and engineer assigned to examine the habitat and its umbilical, logged: “The main power cable has dead shorts on two legs and a higher resistance short on the third. Thus, even if we had fixed the gas leaks, we probably would have had to abort anyway.”
Cannon on the second dive wore a hot-water suit with no insulation under it, no gloves and no hood, and his task was to examine the leaking penetrators, which are where the cables enter the hull, under a cover plate, in a curtain of bubbles. Hardy argues that to see them he had to touch the habitat. Barth, under the hatch a few feet away, wore neoprene gloves and a hood. Hardy’s reconstruction is that Cannon, grounded to the hull by bare hands and head, became part of the circuit. Blackburn and Reaves heard a scream through the water. Hellwarth, who reported the scream in his own book, agrees that it “would presumably be a much more likely reaction to a sudden electrical shock than to a surreptitious poisoning from a buildup of CO2.” His jaws were clenched so hard that neither Barth nor Blackburn could open them. Electric shock drowning, the deaths every year of swimmers in marinas with leaking shore power, is a recognised pattern, and “in the vast majority of Electric Shock Drownings, the victim’s autopsy shows no signs of electrical injury.”
Ben Hellwarth, whose 2012 history of Sealab is the fullest account of the programme, wrote to the magazine that published Hardy’s article that the board’s theory “always had a number of holes in it,” but that he found Hardy’s “camera angle to be fuzzy.” Hardy replied at length, with letters from five of the divers, among them Blackburn, who wrote that he agreed with electrocution and that there was “NO empty canister at start of dive number 2,” and Harrell, who described the ground faults and a badly repaired power connection on the habitat. In 2025 Hardy asked the National Archives for the board’s report and transcripts, and was told that its records of boards of inquiry end in 1953.
What the record supports
That the Mark IX had not been breathed from below sixty feet before 16 February, and that its last modifications had not been tested at depth. That two men came back from the first dive unable to breathe easily and not thinking clearly. That the rigs were refilled by the most careful man on the ship, carried down by a man who felt no light one, and then hung outside the capsule in the sea for half an hour, where the procedures never contemplated their being. That the habitat’s main cable was faulting to its hull before the dives and shorted after them, and that the hull was the ground. That Cannon worked at the cable penetrators without gloves. That a scream was heard. That the analysis of the air in his trachea was made many hours after death, and that a congressman could say in July that the cause had never been determined, and be right.
Sources: Kevin Hardy, “SEALAB III (1969): The Divers’ Story,” Journal of Diving History 32 (119), 2024, carrying Blackburn’s note, Moore’s, Harrell’s and Pruna’s recollections, the Talkington and Hallanger logs, the passages from the habitat’s operating procedures and from Ocean Industry, May 1969, and the quotation from Bond’s Papa Topside; Kevin Hardy, “SEALAB III: Another Letter to the Editor,” InDEPTH, 2024, with the divers’ replies; the Long Beach Independent and Tampa Times reports transcribed on Islapedia; Time, 28 February 1969; the Congressional Record, 15 July 1969; and Hardy’s request on History Hub, 2025. All are linked from the entry’s sources.