Update (13 May, 2019): A previous title image showed USS Pompanito but has been changed to show USS San Francisco. 

A nuclear sub, a hidden underwater mountain, and a deadly collision. The story of USS San Francisco sounds like the plot to a Kathryn Bigelow epic, or a deep-sea Titanic. Remarkably, unlike the Titanic the submarine didn't sink.

At 11.42am local time on January 8, 2005, crew aboard the nuclear submarine felt a violent jolt as the vehicle, cruising along at top speed (around 30 knots, 55 km/h or 35 mph), crashed head-first into a seamount. The sub was approximately 580 kilometers (360 miles) southeast of Guam and 160 meters deep (525 feet) when it struck.

The scene was pandemonium. One officer described it as "a slaughterhouse" – bodies were flung distances of 6 meters (20 feet), blood covered the cabins, and sailors lay on the floor unconscious and injured. 

"I thought I was going to die," the sub's captain, Commander Kevin G Mooney, told The New York Times.

Recollections of the day's events describe a makeshift emergency room, with ripped shirts serving as DIY bandages and cleaning brushes subbing for splints, as the moderately injured attended to colleagues and friends presenting severe (some life-threatening) wounds. 

It turned out that the charts used by the crew to navigate the seas omitted the presence of a seamount and the pilots of the sub were unaware that they were heading straight for an underwater mountain. That mistake cost the Navy $88 million (or more) in damages – and a life.

Machinist’s Mate 2nd Class Joseph Ashley was in the sub's smoking room when he sustained fatal injuries. Medics were unable to nurse the wounds in time and he died as a result. 

Mooney has since accepted full responsibility for the tragedy, recognizing that several mistakes were made – the sub was going too fast, for example. The crew had taken inadequate depth soundings and, of course, they hadn't cross-referenced the navigational chart with others that would have highlighted the existence of underwater obstacles. 

But it could have been worse.

The sub did not sink. There was no reactor malfunction. And all but one of the staff onboard survived the collision. The sub even made its way back to port in Guam – on its own power.

This was only possible because of the SUBSAFE program, an initiative the Navy introduced in the wake of a similar disaster 42 years prior. 

In 1963, USS Thresher left Maine to begin a series of deep-sea tests. After reaching the designated test depth, the crew made contact with an accompanying submarine rescue ship (USS Skylark), but the communications were muffled.

Listeners described a strange sound, "like air rushing into an air tank". Then, the line went silent. There were 129 souls aboard the submarine and not one made it out alive.

The purpose of SUBSAFE was to ensure that whatever happened, the submarine's hull would keep its structural integrity under pressure and the vehicle can surface. Additional safety precautions were introduced to protect the nuclear reactor and ballast systems. 

These actions are what stopped USS San Francisco from sinking and allowed it to surface and move back to base. Indeed, even though it sustained severe damage, USS San Francisco lived on to fight another day, only retiring in 2016. 

[H/T: The New York Times, Popular Mechanics,]