During the Cold War, the U.S. was desperate to learn about Soviet Intercontinental Ballistic Missiles and Russia's nuclear first-strike capabilities.

Then, in the early 1970s, the U.S. learned of the existence of an undersea communication cable that connected the Soviet Pacific Fleet naval base at Petropavlovsk to the fleet's mainland headquarters at Vladivostok.

Operation Ivy Bells location Source: Nzeemin/Wikimedia Commons

The cable ran under theSea of Okhotsk, between the Kamchatka Peninsula and the Russian mainland. Russia considered the Sea of Okhotsk as part of its territorial waters, and entry was forbidden to foreign vessels.

To ensure that their sovereignty wasn't violated, the Soviet Navy installed a network of sound detection devices on the sea floor that was designed to detect intruders.

Captain James Bradley's four challenges

In 1966, James F. Bradley Jr. became Undersea Warfare Director for the U.S. Office of Naval Intelligence. In 1968, he led the mission that sent a submarine, the USS Halibut, to search the Pacific for the downed Soviet submarine K-129.

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In 1970, Bradley decided to tap into the newly identified Soviet undersea cable, and thus was born Operation Ivy Bells. Bradley believed that the Soviets hadn't bothered to encrypt the cable because it was buried so deep underwater, and it was so close to Russia's coast.

His first challenge was to find a way to pay for a mission to tap the cable. In 1970, the U.S. Navy was working on a deep-submergence rescue vehicle (DSRV) program, which was a vehicle that could rescue submariners in the event of an accident. The Navy diverted funds from this program, and outfitted the Halibut with something that looked like a DSRV vehicle, but was actually what's known as a diver lockout, and was called the "Bat Cave".

DSRV-1 (Mystic) docked to a Los Angeles-class attack submarine Source: U.S. Navy/Wikimedia Commons

Bradley's second challenge was to figure out a way for divers to stay at a depth of 400 feet for the several hours that it would take to install a wiretap on the cable. His answer was helium.

At the surface, you and I breathe a mixture of approximately 80% nitrogen and 20% oxygen. Since the 1950s, U.S. Navy Captain George F. Bond had been experimenting with gases that would allow divers to reach much deeper depths and stay for longer periods.

When the nitrogen and oxygen in our blood are compressed by water pressure, the nitrogen builds up in the blood, causing the dangerous condition called nitrogen narcosis, or decompression sickness, and fatal embolisms if decompression is done too quickly.

DSRV accommodation chamber Source: Soham Banerjee/Wikimedia Commons

Instead of breathing nitrogen, participants in Operation Ivy Bells would breathe helium along with oxygen. This was one of the first uses of saturation diving. Because helium has a lower molecular weight than nitrogen, it exits human tissues more quickly, and that made it perfect for the task.

Bradley's third challenge was finding the Soviet cable in over 600,000 square miles of water. The answer came to him while he was remembering his boyhood on the Mississippi River. He remembered signs being posted on the shore warning boaters not to anchor because there were utility lines at the bottom of the river. Bradley reasoned that if they used those signs in America, the Russians would use something similar.

Sure enough, when Halibut reached the Sea of Okhotsk in October 1971, her sailors scanned the shoreline and found signs warning fishermen to avoid the area where the cable was sunk.

Bradley's fourth challenge was how to actually tap into the cable without shorting it out. His answer was induction. A 20-foot (6.1m) long device was designed to wrap around the 5-inch-wide cable without piercing its casing. Also, the device was designed to automatically fall off if the cable was ever raised for repair.

A cover story that turned out to be true

Ready to go in the Fall of 1971, the Navy needed a cover story to account for the presence of Halibut in the Sea of Okhotsk, and they came up with one that was so good, it turned out to be true.

The story was that the Halibut was sent to search for and recover debris from a Soviet SS-N-12 Sandbox supersonic anti-ship missile (AShM) so that the U.S. could create countermeasures.

In all, U.S. Navy divers recovered more than two million pieces of missile debris that were taken back to the U.S. Naval Research Laboratory where they were reconstructed, and the missile was reverse-engineered. The Navy discovered that the missile had been guided by radar only, and not infrared as had been thought.

An almost 10-year run

From October 1971 on, every month, divers emerged from the Halibut or a sister ship, they would retrieve the recordings and install new tapes. Then, the tapes were taken to the National Security Agency (NSA) for processing.

The mission was so successful that a nuclear-powered tap that could store a year's worth of data was created for the Navy by AT&T's Bell Laboratories, and additional submarines began monitoring Soviet undersea cables around the world. For almost ten years, everything was going swimmingly (pun intended), then disaster struck.

In January 1980, a 44-year-old employee of the NSA named Ronald Pelton realized he was $65,000 in debt. Pelton walked into the Soviet embassy in Washington, D.C. and he sold the secret of Operation Ivy Bells to the Soviets for $35,000.

The U.S. was taken by surprise when, in 1981, U.S. surveillance satellites showed a flotilla of Soviet ships, including a salvage vessel, sitting right above the site of the wiretap. The submarine USS Parche was dispatched to recover the tap, but they found that the Soviets had already taken it. As of 1999, the wiretap device was on public display at the Great Patriotic War Museum in Moscow.

In July 1985, the reason the tap had been discovered was revealed when Pelton's contact, a KGB colonel named Vitaly Yurchenko defected to America. Yurchenko told U.S. agents about Pelton's espionage, which led to Pelton's arrest and 29-year jail sentence. He was released in 2015.