The I-401 submarine, one of a pair captured a week after Japan surrendered in 1945, is from the Imperial Japanese Navy's I-400 Sensuikan Toku class of submarines. They were the largest built before the nuclear ballistic missile submarines of the 1960s.

The U.S. Navy brought both Sensuikan Toku subs back to O'ahu and sunk them a few months later after Soviet scientists began demanding to examine the 400-foot-long hulks, which were nearly 40 feet high and could hold a crew of 144.

Researchers with the undersea team, which happened upon the I-401 during test dives, said raising the sub from almost 2,700 feet below the sea surface and towing it ashore would likely cost tens of millions of dollars.

"I don't imagine anyone would want to do anything like that," said John Smith, science program director at the Hawaii Undersea Research Laboratory. "It's down deep and big. And it's broken. The bow section is torn off and there's a large debris field."

The team is planning a dive for as early as August after researchers return from an international study of undersea volcanoes along the Kermadec Trench in the South Pacific.

Historians from the National Oceanic and Atmospheric Administration will also accompany the dive, Smith said.

The Japanese sub is just one of thousands of sunken ocean, air and land craft, most from the past two centuries, in the waters around the Hawaiian Islands.

Beneath the ocean swells are subs and ships scuttled by the military, old whaling and merchant ships, fishing boats, landing craft, trucks and 20th century recreational craft. Most are submerged near Pearl Harbor and old plantation-era landing sites, such as Lahaina and Kahului, Maui; Honolulu; and Hilo, Hawai'i, said Hans Van Tilberg, maritime heritage coordinator for NOAA Marine Sanctuaries Program.

"Every time those guys in the HURL submersible go down, they come across something new," Van Tilberg said.

Two days earlier, the team had found a scuttled American S-19 submarine and in 2002 happened upon a Japanese midget submarine torpedoed during the 1941 attack on Pearl Harbor.

Finding information on wreck locations in Island waters takes some scavenging, as there are no comprehensive databases or maps of sunken objects. The Coast Guard, NOAA, recreational diving companies and Naval Historical Center in Washington all keep partial lists of Hawai'i's marine debris, Van Tilberg said.

Van Tilberg said he has also consulted old editions of Hawai'i newspapers and journals, which kept meticulous track of lost ships in their maritime columns.