A private dive team exploring the waters off south-central Alaska has discovered the oldest American shipwreck ever found in the state, officials said Monday.

The Torrent sank 139 years ago in Cook Inlet after tidal currents, among the world's most powerful, rammed it into a reef south of the Kenai Peninsula. Documents from the period show that all 155 people on board survived.

The U.S. had purchased Alaska from Russia for $7.2 million in 1867, less than a year earlier, and about 130 U.S. Army soldiers had come north on the Torrent to build the first U.S. military fort in south-central Alaska, now the state's most populous region.

"It's a very significant find because it's right after the purchase, during the transition from Russian to American authority," said Judy Bittner, a state historic preservation officer. "It's the very beginning of federal presence in Alaska and the establishment of order."

About 20 sailors and 15 of the soldiers' wives and children were also on board.

A four-man dive team led by Steve Lloyd, owner of Anchorage's largest independent book store, found remnants of the wreckage in July. Until last week, they kept the discovery secret at the request of state officials, who wanted more time to document the site before any looters arrive.

"The actual depth of wreck site is still classified by state authorities," Lloyd said. "We have by no means found everything."

An array of objects, from guns, cannons, shoes and plates, are hidden beneath the broad leaves of giant kelp beds or concealed in caverns and crevices among massive boulders, Lloyd said.

"It's like walking through a field of tall grass and undergrowth looking for a baseball that you've lost," Lloyd said.

Big finds include the two anchors, sections of hull and heavy bronze rudder hinges weighing about 100 pounds (45 kilograms). The objects lie scattered across an area nearly 300 yards (meters) from the main wreck site. The team managed to map a section measuring 200 by 150 feet (60 by 45 meters).

The search cost about $2,000 (euro1,419), Lloyd said.

About 2,500 ships have wrecked off the Alaska coast since Russian explorers first arrived in 1741, according to Mike Burwell, a cultural anthropologist for the federal Minerals Management Service. A partial database on the service's Web site lists Japanese submarines and fishing trawlers, Liberian freighters and New England whaling ships, among others.

The oldest known American shipwreck in Alaska is the Eclipse, a Yankee fur trading vessel. It sank in the Shumagin Islands on August 11, 1807, south of the Alaska Peninsula, and has never been found, Burwell said.

The Torrent is now being considered for listing in the National Registry of Historic Places. Bittner said state or federal archaeologists may study the wreck if they can secure enough funding.