A U.S. nuclear submarine that ran aground over the weekend appears to have struck a natural feature on the floor of the Pacific Ocean, a Navy spokesman said Monday.

Lt. Cmdr. Jeff Davis said an initial investigation turned up nothing to indicate the USS San Francisco (search) struck anything but a large rock, land or other natural feature as it conducted underwater operations about 350 miles south of Guam (search).

The ship "struck something very hard and did an emergency surface," Davis said at a media briefing in Guam.

Davis added there were no reports of damage to the submarine's nuclear reactor, and the vessel made its way back to its home port in Guam Monday under its own power. The submarine's outer hull was damaged, but its inner hull remained intact.

The submarine had been headed to Australia (search) for a port visit.

One sailor was killed and at least 23 others suffered injuries including broken bones, cuts and bruises, the Navy said. The submarine has a crew of 137.

Machinist Mate 2nd Class Joseph Allen Ashley, 24, of Akron, Ohio, died Sunday after suffering major head injuries, Davis said. He had been working in the sub's engineering compartments.

Adm. Thomas Fargo, commander of U.S. Pacific Command based in Hawaii, said Monday a "rapid and thorough" investigation was under way.

Fargo, who had a prominent role following one of the more notable Pacific submarine collisions in recent years, declined to speculate on the cause of the weekend accident.

As commander of the Pacific Fleet, Fargo presided over a rare court of inquiry following the Feb. 9, 2001, sinking of a Japanese fishing boat by the nuclear-powered attack submarine USS Greeneville. Nine men and boys were killed in the collision.

Fargo found that the Greeneville's skipper rushed his crew through preparations for a surfacing exercise and performed an insufficient periscope sweep. The commander was reprimanded and agreed to retire from the Navy.

The San Francisco is one of three submarines based on Guam. Located west of the international dateline, Guam is a U.S. territory about 3,700 miles southwest of Hawaii.