The discovery of wreckage from the Argentine submarine ARA San Juan in November 2018 grimly highlights the dangers inherent to submarine operations even in peacetime. Well over a dozen submarines have been lost catastrophic accidents since the end of World War II. Only stringent safety protocols and rigorous maintenance regimes can minimize the likelihood of such accidents.

Fortunately, though U.S. submarines have experienced numerous collisions and groundings, it has been over fifty years since the U.S. Navy lost its last submarine to causes which remain unclear to this day—though lax safety procedures may well have been involved.

The USS Scorpion was one of six Skipjack-class nuclear-powered attack submarines, a class which introduced a tear-drop shaped hull that enabled it to attain high speeds of thirty-four knots (thirty-eight miles per hour) while submerged.

Launched by Electric Boat in Connecticut in 1960, Scorpion operated as part of the Atlantic submarine fleet from her base in Norfolk Virginia. She helped test new nuclear submarine tactics and was also involved in clandestine missions, including reputedly infiltrating Soviet waters to spy on the launch of a new missile type in 1966.

By February 1967, it came time for Scorpion to undergo extensive routine testing and overhaul as part of the Navy’s SUBSAFE safety program. This required thoroughly testing surfaces and components using ultrasounds to certify the vessel’s integrity for continued operations.

SUBSAFE was devised after the loss in 1963 of the USS Thresher, the lead ship of a more advanced successor to the Skipjack class. An apparent rupture in her pipes allowed saltwater to spray into the vessel, causing a chain reaction leading to a reactor shutdown, a failure of the air flasks used to surface, and the progressive flooding of the submarine. The Thresher sank with 129 aboard—amounting to the deadliest submarine accident ever.

However, SUBSAFE quadrupled submarine overhaul times. Because demand for submarine operations was so high, the Chief of Naval Operations authorized Scorpion to receive an “experimental” accelerated overhaul, bypassing required safety checks and installation of improved, safer components. This despite over-worked repair crews relying on jury-rigged spare parts and discovering numerous defectively welded pipes. These shortcomings forced the Scorpion to observe a much shallower-than usual diving limit of 110-meters.

As a result, Scorpion was back in operational status seven months later—one of only four in the Atlantic Fleet not to receive her SUBSAF certification. The poor condition of the Scorpion after the overhaul was described by two Scorpion crew members Andy Elnicki and Dan Rogers—both of whom left the crew before her final voyage.

In February 1968, Scorpion transited to a naval base in Rota, Spain. Crew letters reveal that she suffered mechanical breakdowns including a leaky seal on its propeller shaft, major oil leakage from its hydraulics, unstable control surfaces, leaks of poisonous Freon, and an electrical fire near her Trash Disposal Unit.

Nonetheless, on May 16 Scorpion sortied from Rota, tasked with ferreting out the position of Soviet ships reported near the Azores islands, before returning to homeport at Norfolk. Approaching midnight on May 21, the Scorpion’s captain, thirty-six-year-old Commander Francis Slattery, transmitted that he had located a Soviet submarine at a depth of 110 meters, and was “to begin surveillance of the Soviets.”

This was the last message received from the ninety-men aboard the Scorpion.

After failing to return to port, the U.S. Navy began searching in June for its missing submarine, employing dynamic Bayesian statistical methods to optimize the search pattern. The search area was further narrowed based on an ominous recording obtained by a Navy listening station in the Canaries, which captured fifteen acoustic events over 190 seconds. These seemed to correspond to an apparent underwater explosion or implosion.

After four months, the oceanographic search and rescue vessel USNS Mizar discovered the 77-meter long Scorpion’s wreck 460 miles southwest of the Azores using a jury-rigged underwater towed camera-sled.

Located 3,000 meters deep—well below the Skipjack-class’s test depth of 210 meters—the Scorpion had plowed a deep trench across the ocean floor, imploded by the deep pressure of the surrounding ocean water. The submarine’s operation center had collapsed inward, causing the sail (or conning tower) located above it to tear off.

The mystery of what brought the Scorpion to her terrible fate in May 1968 has never been satisfactorily resolved—inspiring numerous books offering competing explanations.

A naval court of inquiry made public in January 1969 determined that the cause of the Scorpion’s demise could not be ascertained, though decades later it was revealed it had privately concluded detonation of the 330-pound warhead on a defective Mark 37 torpedo was the most likely explanation. Had the stubby, sonar-guided anti-submarine torpedo launched accidentally—or been jettisoned—and then circled around to sink the only target in the area?

The Mark 37 was also infamous for using silver-zinc batteries prone to overheating and even combusting or exploding. A theory later expounded in the book Blind Man’s Buff is that a “hot-running” torpedo battery led to a fire, causing a torpedo explosion which ruptured the hull.

However, another study conducted by naval acoustic specialists rejected the explosion explanation, citing the lack of an evident “bubble pulse” usually produced by an underwater explosion. According to this analysis, the Scorpion had already begun sinking uncontrollably due to other causes prior to the pressure-induced implosion recorded by the acoustic listening station.

One theory advanced by Vice Admiral Arnold Schade and former-Scorpion mechanic Dan Rogers postulates it sank due to a faulty Trash Disposal Unit—a valve on the galley for getting rid of waste. This could have allowed seawater to flood in and contaminate the batteries, producing hydrogen gas that may have incapacitated the crew and eventually led to the apparent explosion in the center of the Scorpion’s hull.

A variation on this explanation is that the Scorpion suffered a hydrogen explosion while charging batteries at periscope depth. Basically, while adhering to safe “total” cell voltages, the charging process sometimes channeled excessive energy into individual cells—which in extremis, could release hydrogen gas sufficient to cause a hull-rupturing explosion. Over-charged individual cells were reportedly a common problem in U.S. submarines at the time.

One sinister theory expounded in several books—All Hands Down, Red Star Rogue and Scorpion Down—maintains the Scorpion was sunk by a Soviet submarine or helicopter-launched torpedo. Two months earlier on March 8, Soviet ballistic missile submarine K-129 sank mysteriously, taking with her all 98 hands aboard—and many Soviet naval officers were convinced she had been rammed by a trailing U.S. submarine. Might the Soviets have conspired to bushwhack the Scorpion in revenge—or even accidentally collided or torpedoed her while tailing from behind?

According to this theory, the Soviet Navy might have used intelligence transmitted by the infamous John Walker spy ring to determine the Scorpion’s position. Afterward, both navies supposedly colluded to hush up the two incidents to prevent political repercussions. However, these theories rely on conjecture, anonymous sources, and a presumed cover-up rather than concrete evidence. Officially, U.S. Navy maintains no Soviet ships were within 200 miles of the Scorpion when she sank.

The Navy still periodically surveys the Scorpion’s wreck to check for radioactive leakage from her S5W nuclear reactor, and the two ASTOR nuclear-armed torpedoes still residing in her hull. As recently as 2012, the Navy has rejected calls to re-investigate the tragic sinking.

It seems unlikely the exact truth of the Scorpion’s fate will ever be known for certain, but her undersea grave stands in mute testimony to the high price submariners from many nations have paid pursuing their dangerous line of duty.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.

Image: Wikimedia Commons.