Here's What You Need To Remember: The bow of sister submarine USS Honolulu, soon to be retired, was removed and welded onto San Francisco. The submarine rejoined the fleet in 2009 and served for another seven years.

USS San Francisco is a Los Angeles-class nuclear attack submarine. Submarine builder Newport News Shipyard began construction on it in 1977, and it was commissioned on April 24, 1981. The submarine joined the U.S. Pacific Fleet and served there throughout its career.

Like all Los Angeles-class submarines, it displaced 6,900 tons submerged, was 362 feet long, and had a beam of 33 feet. A General Electric PWR S6G nuclear reactor provided 35,000 shipboard horsepower, driving the submarine to a speedy 33 knots. A typical crew consisted of 129 officers and enlisted men.

On January 8, 2005, the USS San Francisco was traveling at flank (full) speed—approximately 38 miles an hour at a depth of 525 feet. It was 360 miles southeast of Guam heading to Brisbane, Australia for a liberty stop. Navigation plotted the route based on undersea maps that were generally agreed to give the most complete view of the seabed. According to the New York Times, the captain went to lunch and the navigation officer, believing it was safe to do so, dived the submarine from 400 to 525 feet and accelerated to flank speed.

At approximately 11:42 local time, while transiting the Caroline Islands mountain chain, the submarine came to an abrupt—and unexpected—halt. There was a shudder and then a tremendous noise. Men throughout the ship were thrown from their stations against their surroundings. In an instant many suffered bruises, lacerations, broken bones and fractures. A chief petty officer described the scene as looking like a “slaughterhouse,” with blood running everywhere. Ninety-eight crewmen were injured with one, Machinist's Mate Second Class Joseph Allen Ashley, fatally injured.

Despite their injuries, and not having any idea what had just happened, the captain and his crew rushed to surface the boat. The crew threw the emergency blow activator, known as the “chicken switch,” that immediately blast compressed air into the USS San Francisco’s ballast tanks. Unknown to the crew, the impact of the explosion had punched huge holes in the forward ballast tanks. The submarine was supposed to immediately rise, but it was an agonizing thirty seconds before the submarine began to surface. By 11:44 the submarine had surfaced.

Damage control reported the USS San Francisco’s inner hull was intact, its Mk. 48 torpedoes and Tomahawk cruise missiles were unharmed, and remarkably, its nuclear reactor was completely undamaged. All alone in the Pacific, the submarine began the long trip back to Guam. The submarine limped back into Apra Harbor in Guam thirty hours later on January 10, the crews of other moored submarines manning their rails in the stricken submarine’s honor.

Later, an investigation would reveal the submarine had crashed into a seamount rising 6,500 feet from the ocean floor. The seamount had not appeared on the charts that San Francisco’s crew had used to plot their course, but appeared on other charts as a “potential hazard.” The hazard was reported two miles from the site of the collision and the Captain of the San Francisco has stated that had he known about it, he would have given the potential obstacle a wide berth.

The chart used by the USS San Francisco’s crew was prepared by the Defense Mapping Agency in 1989. According to a study of the incident prepared by the University of Massachusetts in 2008, a Landsat satellite image showed a seamount in the area of the collision that rose to within one hundred feet of the surface. The Navy’s charts were not updated with the new data—according to the UMass report, the Navy believed that with the cessation of the Cold War the crash site area was not a high priority for mapping, and that priority had instead been given to the Middle East region to support the Global War on Terror.

After repairs to ensure hull integrity, San Francisco traveled under its own power to Puget Sound, Washington. The damaged portion of the boat’s bow was removed. The bow of sister submarine USS Honolulu, soon to be retired, was removed and welded onto San Francisco. The submarine rejoined the fleet in 2009 and served for another seven years. In January, it began a two-year conversion that will turn it into a permanently moored training submarine.

The heroic actions of the crew were essential to the submarine’s survival. Still, how did a submarine survive a high-speed collision with a mountain? In 1963, immediately after the loss of USS Thresher, the Navy instituted the SUBSAFE program. The goal of the program was to ensure that a submarine’s hull would retain pressure in the event of an accident and it would be able to surface. The Navy’s Nuclear Propulsion Program made safe, resilient nuclear reactors an absolute top priority.

If a submarine’s hull remained intact, then it was able to surface and the reactor continued to operate the crew had a shot at survival. The USS San Francisco was able to do all three. That it was able to survive was no accident but rather the culmination of decades of hard work and dedication by the U.S. submarine force.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This piece was originally featured in 2016.

Image: Reuters.