If ever there was an ultimate weapon of war, it would have to be the nuclear submarine. For more than 60 years, ballistic missile-armed submarines have prowled the world’s oceans, ready to unleash nuclear armageddon at a moment’s notice. Meanwhile, fast attack boats track and stalk the missile-armed ‘boomers’, the two rivals locked in a shadowy game of cat-and-mouse deep beneath the waves. But while nuclear deterrence is the modern submarine’s most famous mission, it is far from the only one. Throughout the Cold War and into the present day, these vessels have served as the eyes and ears of the Navy, using stealth and guile to perform covert reconnaissance of enemy shores. And perhaps the most daring and successful such feat of naval espionage took place in the early 1970s, when a U.S. Navy submarine snuck into a Soviet naval base to tap an underwater communications cable. This is the extraordinary story of Operation Ivy Bells.

Our story begins in the late 1960s at the Petropavlovsk Naval Base on the west coast of the Kamchatka Peninsula, home of the Soviet – and today Russian – Pacific nuclear submarine fleet. To connect the base to the Pacific naval headquarters at Vladivostok – and the rest of the Soviet military communications network – the Soviets laid a submarine cable more than 1500 kilometres across the sea of Okhotsk. From a security standpoint this was a wise decision, for unlike radio traffic, communications could not be intercepted without gaining physical access to the cable. And with the area heavily defended by acoustic sensors, mines, and regular naval patrols, this was highly unlikely.

Or so the Soviets thought, for on the other side of the Pacific, one man was planning to do just that: Captain James F. Bradley Jr., chief of the highly secret undersea warfare division of the U.S. Office of Naval Intelligence. If, Bradley reasoned, the sea of Okhotsk could be infiltrated and the Petropavlovsk-Vladivostok cable found and tapped, it would yield a bonanza of valuable intelligence on Soviet naval operations in the Pacific. But how to carry out such a daring mission? Thankfully, the U.S. Navy had the perfect tool at its disposal: the U.S.S. Halibut.

Laid down in 1957, the Halibut started out as a diesel-electric submarine but was converted to nuclear power halfway through construction. Shortly after her commissioning in 1960, she became the first submarine in history to launch a guided missile. However, unlike the later submarines Halibut was designed to launch the SSM-N-8 Regulus, a jet-powered, nuclear-armed cruise missile. Five Regulus Is – or two of the later Regulus IIs – were stored in a special, 27-metre-long cylindrical hangar built into the Halibut’s forward hull, and were extracted and deployed onto her deck by automatic hydraulic equipment. Unfortunately, missiles could only be launched while the submarine was surfaced, making her highly vulnerable to detection and attack.

By 1965, Regulus-armed vessels had been rendered obsolete by the more advanced George Washington-class ballistic missile submarines, and Halibut was instead modified for use in “Underwater Engineering” – a euphemism for covert intelligence gathering. The now-empty Regulus missile hangar became the nerve centre of the vessel, housing a state-of-the-art Sperry UNIVAC 1224 and the Intelligence personnel who would operate it. This space became known as the “Bat Cave”. The computer was connected to an extensive suite of sensors, including remotely-operated vehicles or ROVs and a special sled or “fish” that could be towed behind the submarine and scan the seafloor with sonar and cameras. Halibut was fitted with skids called “sea keeping legs” or “skegs”, larger anchors, and position-keeping thrusters so she could rest on the seafloor or hover just above it while deploying the ROV or divers. Funds for these modifications were secretly drawn from the Deep Submergence Rescue Vehicle or DSRV project, and in 1971 a torpedo-shaped device officially designated a DSRV crew trainer was attached to Halibut’s aft deck. In reality, this structure was actually a pressurized habitat for the newly-developed technique of saturation diving.

When a diver remains underwater for long periods of time, nitrogen from the air they breathe becomes dissolved in their tissues. If they then surface too quickly, this nitrogen can bubble out of solution, leading to the dreaded decompression sickness or the bends – and for more on this, please check out our previous video Why is it Called The Bends When There is No Bending, and the Most Gruesome Accident. The standard solution to this problem for divers to surface slowly, allowing the nitrogen to be slowly and safely expelled from their bodies. However, when working at great depths, this process can take hours; and if a diver must make multiple dives over a given time period, the time spent decompressing becomes impractical, and multiple decompressions expose the diver to unnecessary risk of the bends. In the late 1950s, however, Captain George F. Bond, a U.S. Naval Physician, came up with a clever solution. After a certain amount of time spent working at depth, a diver’s body becomes saturated with nitrogen, such that no matter how much longer they remain underwater, their total decompression time remains the same. Thus, if the diver can be kept at this same pressure throughout their working shift, they only need to decompress once at the end, maximizing productivity and minimizing the risk of decompression sickness. In 1964, Bond tested his theories by building SEALAB I, a pressurized “habitat” sunk 59 metres below the surface off the coast of Bermuda. For 11 days, 4 divers lived aboard the habitat, venturing out to conduct scientific experiments and evaluate the function of the diving equipment. This was followed by SEALAB II in 1965 and SEALAB III in 1969, both of which demonstrated the practicality and utility of the saturation diving technique. Today, saturation diving is used extensively in the offshore oil and gas industry – and to find out just how dangerous this technique can be, please check out our previous video The Most Gruesome Death Imaginable: the Byford Dolphin Accident.

The refitted Halibut’s first clandestine mission was to find and photograph the wreck of the Soviet Golf II-class submarine K-129, which had sunk with all hands 2,890 kilometres northwest of Hawaii on March 8, 1968 This mission was part of the larger Project Azorian, wherein the CIA constructed a special ship called the Glomar Explorer to snatch K-129 off the ocean floor. The cover story for the mission was that Glomar Explorer was constructed by eccentric billionaire Howard Hughes to mine manganese nodules off the ocean floor – a scheme crazy enough to be plausible. Incredibly, in July 1974, Glomar Explorer succeeded in recovering the forward section of K-129 from a depth of 4.9 km – but that is a story for a different video.

In October 1971, Halibut sailed from Hawaii on a covert mission to find and tap the submarine cable between Petropavlovsk and Vladivostok, code-named Operation Ivy Bells. While finding a single 12-centrimetre-wide cable in thousands of square kilometres of ocean should have been an insurmountable task, Captain Bradley, the architect of the mission, had come up with a laughably simple solution. Having grown up in St. Louis Missouri, Bradley remembered seeing signs along the Mississippi River reading CABLE CROSSING – DO NOT ANCHOR. There must, he reasoned, be similar signs around Petropavlovsk to prevent local shipping from snagging and damaging the cable. And indeed, shortly after arriving in the Sea of Okhotsk, the crew of the Halibut spotted such a sign on the shoreline and quickly located the cable in 120 metres of water. Operating from their pressurized habitat on Halibut’s aft deck, U.S. Navy saturation divers fitted the cable with a special signal-interception pod developed by the NSA and Bell Telephone Laboratories. 6 metres long, the pod did not penetrate the cable but gathered signals remotely through electromagnetic induction. The device could record up to eight weeks worth of signals and was designed to automatically drop off if the Soviets raised the cable for regular maintenance.

Halibut’s primary mission was so secret that few of her crew were aware of it. Instead, they believed they were sent to collect wreckage of Soviet SS-N-12 Sandbox anti-ship missiles test-fired over the Sea of Okhotsk. In fact, this cover mission was actually successfully carried out, with enough debris being collected for the missile to be fully reverse-engineered and evaluated for weaknesses.

With the tap successfully installed, Halibut slipped out of Soviet waters and returned to Hawaii. Every month thereafter, Halibut and other submarines including USS Seawolf, USS Parche, and USS Richard B. Russell returned to Petropavlovsk to recover and replace the recording tapes. To the Navy and the NSA’s delight, the first intercepts revealed that the Soviets were so confident in the security of the cable that nearly all communications sent over it were unencrypted – yielding a treasure trove of intelligence. Buoyed by this success, in 1973 the Navy installed a more sophisticated surveillance device, which was powered by a plutonium-fuelled Radioisotope Thermal Generator or RTG, could record 12 communications channels at once, and could record up to a year’s worth of data. And five years later, another tap was successfully placed on the cable connecting the Severdovinsk naval base to the Soviet Northern Fleet headquarters in Murmansk.

But this intelligence windfall came to an abrupt end in 1981 when a U.S. spy satellite spotted a small fleet of Russian ships gathered over the exact location of the recording device. Alarmed at this development, the Navy dispatched the U.S.S Parche to recover the next set of tapes. But when the submarine reached the site, they discovered that the tap had been removed. Operation Ivy Bells was over. But the project’s downfall was not the result of superior Soviet spy craft; rather, it had been betrayed from within. In January 1980, a 44-year-old former NSA analyst named Ronald W. Pelton walked into the Soviet Embassy in Washington D.C. and offered to sell the KGB everything he knew. At the time, Pelton was $65,000 in debt, had just filed for personal bankruptcy, and had only a few hundred dollars left in his bank account. In addition to Ivy Bells, Pelton disclosed the details of at least seven other NSA operations, relying on his excellent memory instead of physical documents. For his services, he received just $35,500 – not even enough to cover his debts. Then, in July 1985, Vitaly Yurchenko, a KGB Colonel and Pelton’s initial contact, defected to the United States and provided information which eventually led to Pelton’s arrest. The following year, Pelton was convicted of espionage and sentenced to three consecutive life sentences in prison – where he eventually died of natural causes on September 6, 2022. Thankfully, Pelton was not aware of the Severdovinsk-Murmansk cable tap, which remained in place until the end of the Cold War in 1991.

Operation Ivy Bells remains one of the greatest and most ingenious intelligence coups of the Cold War – at least, that we know of. Given how many covert operations remain classified to this day, who knows what other daring exploits the “Silent Service” performed during the most dangerous period in human history?

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