The U.S. Navy freighter sits upright on the bottom of the sea 16 miles off Boston, its bow firmly buried in the muddy bottom, its stern exposed, decades of silt covering everything. The charred remains of the 130-foot-long steel ship remains almost undisturbed since a horrific fire engulfed it, causing the deaths of 17 of the 31 crewmates on May 11, 1944.

A Marblehead antiques dealer, Chief Boatswain's Mate Louis Tremblay, was the skipper. The freighter was carrying 150 tons of pyrotechnics, black powder and flat-nosed projectiles that the crew had begun to throw overboard in the munitions dumping ground off Boston when the rockets and the matches that were packaged together ignited, setting off chain-reaction explosions. Many of the men jumped into the frigid waters to escape the massive wall of flames that had engulfed the ship.

Burning debris and thick fog kept rescue ship, the USS Zircon, from getting close enough to pick up survivors for almost 35 minutes.

This little-known page in United States history would have been passed into complete oblivion had it not been for several intrepid divers whose curiosity has compelled them to dive to depths as low as 240 feet searching the hundreds of wrecks strewn over the ocean floor.

Marblehead's Northern Atlantic Dive Expeditions, operated by Heather Knowles and David Caldwell, joined diver Robert Foster and several others who first happened upon the wreck. They knew of a wreck's existence from fishermen whose nets would get tangled in the massive vessel.

"We found something late last fall, but the temperature and weather kept us from diving that deep," said Foster, who lives on the South Shore. "It wasn't until July that we got down and checked it out. We can tell whether an object is manmade or not. It was sharp and steep. We figured it was a ship, but it took a lot of research to find out which one."

Given its radius, the crew from Northern Atlantic Dive Expeditions figured it could have been any one of six or eight ships. Then Knowles and Caldwell found a piece of a compass with a U.S. Navy marking. This was key to uncovering the mystery.

The binnacle, which houses the ship's compass, was engraved with a serial number, a tag stating "U.S. Navy Bureau of Ships," and a date, 1943.

The divers ran down lists of known navy wrecks occurring after 1943. It took about a year, but they narrowed it down to the Navy vessel that sunk on May 11, 1944, the YF 415, but little was known about it.

More research at the National Archives in Washington D.C. and in Waltham finally yielded the jackpot. After submitting a request for information from the government, they received 32 pages of the full Navy inquiry with testimonies of survivors, a description of ways to avoid it and photos of the ship.

The divers compared the report with their own research of the ship. They are convinced their ship is the YF 415.

The tragedy did not have to happen, said Foster. "The Navy invested quite a bit to try to prevent it. Never again would they dispose of live ammo on a power vessel."

Foster said the crew was composed of local men. Skipper Louis Tremblay was a hero. According to a May 13, 1944, story in a Boston paper, he helped a couple of sailors get off the ship. A Salem paper also carried a small story of the tragedy at sea.

When uncovering such mysteries, as much work is done on land as is performed under the sea, said Knowles, a scuba diver since she got "hooked" on it at age 10 by her father, Fred Knowles of Hanson Marine Engineering.

Her group of experienced divers sink deep into the blackness of the ocean floor breathing helium-based gases and undergoing lengthy decompression strategies before resurfacing. Divers dress in dry suits to go into 38-degree waters for 20 to 25 minutes.

What they dream of, said Knowles, is discovering a new shipwreck and uncovering a mystery. She has dived the Andrea Doria, Canada's Empress of Ireland disaster, which claimed 1,000 lives, and other world-class wrecks.

"We love history. We come up from a dive with more questions, and we want to run to the library," said Knowles.

Researching an historically significant vessel like the YF 415 is a way to impart a piece of American history to young people and to connect that history to Marblehead, said the 26-year-old Knowles.