Archaeology students from across the country have delved into the underwater mystery of a steamship that sank more than a century ago off Key Largo.

For two weeks, archaeology students pieced together an underwater puzzle of weathered artifacts -- the metal bands of a paddle wheel, a portion of a smokestack, remnants of boiler plates.

They mapped and measured these artifacts resting on the ocean's floor, trying to solve the mystery of the shipwrecked Menemon Sanford, a grand 237-foot side-paddle steamship that sank in 1862 during a secret Civil War mission.

''At first, all it looks like is a big scattered debris field,'' said Scott Tucker of St. Mary's College of Maryland.

The students came to the Keys from across the country to survey the shipwreck site in Atlantic waters off the coast of Key Largo. It was a field school operated by the nonprofit PAST Foundation in conjunction with the Florida Keys National Marine Sanctuary.

As they worked at 25 feet below the surface, the students also hoped to learn more about the little-known moment in history that involved Union troops, a coral reef and a possible act of sabotage.

''In underwater archaeology, if you have a wreck, you try to document it as best as possible because it could be gone tomorrow,'' said Kathy Schubert, a graduate student of Monmouth (N.J.) University. ``A storm could come through and destroy it or cover it with sand.''

There are about 300 known shipwreck sites in the Keys, with various degrees of documentation. Surveying the Sanford was a ''high priority,'' said sanctuary program coordinator Brenda Altmeier, because of its interesting history that includes a letter to President Abraham Lincoln telling him about the ship's sinking during a war mission.

''Ships grounded all the time back then, but how many are mentioned in a letter to President Lincoln?'' said Anne Corscadden, PAST Foundation project coordinator.

The ship, built in New York in 1854, was named after Captain Sanford, founder of the Sanford Independent Line that ultimately became Royal Caribbean Cruise Line.

The Sanford was the crown jewel of the fleet, built with an oak hull and sheathed with copper. It featured 24 staterooms, with a single-piston engine and two auxiliary masts.

The Sanford survived two accidents, including one on July 5, 1856, when the steamer drove straight into rocks near Boston. The recorded explanation said the captain and officers had celebrated the Fourth of July ``not wisely but too well.''

But the ship's luck finally ran out at 6:40 a.m. Dec. 10, 1862. With the sky clear and the sea calm, the Sanford inexplicably ground its keel into the coral near the Carysfort Reef Lighthouse about four miles offshore of Key Largo.

On board: 800 Union soldiers of the newly organized 156th New York Volunteers on a secret mission to join the General Banks Expedition flotilla rendezvousing in New Orleans.

The U.S. Army's Quartermaster Department had chartered the Sanford, a commercial liner designed to carry only 249 passengers, for $950 a day.

CAPTAIN ARRESTED

''Her fate is surrounded in mystery,'' Corscadden said. ``Because it was such a nice day, and the reef was so visible, they thought it was intentional sabotage and the captain was immediately arrested as a Southern sympathizer. We don't know what happened to him.''

When the Sanford struck the shallow reef, the soldiers began throwing overboard anything not nailed down, including supplies and weapons.

''I think about the panic and everybody trying to get everything off the ship to try to save it,'' Tucker said.

All passengers reportedly survived. The Sanford did not.