In an article you need a subscription to view, the New York Times reports on an email from Rear Admiral Paul F. Sullivan (head of the Pacific sub fleet) describing the USS San Francisco's horrific and enigmatic accident.

The Navy has allegedly confirmed the accuracy of the email.

Unfortunately, Admiral Sullivan does not go into how the accident happened; he simply describes the sad death of Machinist Mate 2nd Class Joseph A. Ashley and states that the San Francisco suffered "nearly instantaneous deacceleration" from 30 to 4 knots.

However, Mr. Ashley's father quotes the sub's Commander as saying he believes they hit an uncharted undersea mountain. The Commander reportedly stated, "On the charts we have, this is a clear area all the way through to Australia."

Gnomon has noted here repeatedly how improbable a Los Angeles-class submarine hitting an undersea mountain is. This accident was caused by either: 1.) Abysmal negligence 2.) Catastrophic equipment failure 3.) An entirely new and suddenly produced sea mount that jutted up so abruptly that the fathometer couldn't pick it up (how likely does that sound?), or 4.) Another craft.

Which solution is more likely we so far lack the data to determine.