One April morning in 1851, after a colossal northeaster had pounded the New England coast, a fisherman found a note in a bottle. It was written by an assistant keeper of Minot Light, a lighthouse on the outermost ledge of the treacherous Cohasset Rocks. Through the howling storm, people on shore more than a mile away had heard the lighthouse bell clanging , but then it had gone silent.

"The light house won't stand over to night," the note read. "She shakes 2 feet each way now."

The lighthouse's uneasy footing, built of iron legs cemented into the granite reef known as Minot Ledge, had struck fear in the hearts of keepers before. According to legend, one of them had a cat that hurled itself into the waves rather than live with the swaying during gales. But during the April storm the structure succumbed to the punishing waves outside Cohasset harbor. Both keepers died.

Yesterday, the Coast Guard launched an expedition to explore the remains of the sunken lighthouse presumed to be on the ocean floor 30 feet below the surface. Divers with special underwater equipment on a Coast Guard cutter plan to map the historic site, left relatively untouched since the completion in 1860 of a new lighthouse made of 3,500 tons of granite.

The divers hope to find clues to why the original lighthouse toppled.

"What really happened? Because nobody knows," said Victor Mastone, director and chief archeologist for the Massachusetts Board of Archeological Resources . "We just know it was there, and suddenly it wasn't, and two men died."

Some local lore has it that the metal remains were salvaged sometime before the Civil War and sold as scrap to a Boston nail maker. And in 1855 the builder of the second tower wrote that he was "determined" to remove the remains he saw through the water.

"But we can find no written records of the salvage," said Mastone.

Yesterday's calm waters provided good diving conditions, but the seas near the Cohasset Rocks are notoriously deadly. Some 19th century mariners called it the most dangerous stretch of New England coastline. Some 40 vessels went down there between 1832 and 1841, according to an engineer looking into the need for a lighthouse more than 150 years ago.

The original Minot Light's spider like design -- with nine legs cemented into 5-foot holes drilled in the rock -- was selected because engineers thought waves would pass through the legs.

Minot Light has long been a source of fascination to historians and lighthouse enthusiasts, in part because the design was scrapped after the accident, and it is one of only two known lighthouse wrecks on the East Coast. Whale Rock Lighthouse, near Narragansett Bay, was felled by a tsunami in 1938. William Thiesen, the Coast Guard historian leading the project's team of 44 crew and divers, said he was struck when he heard about Minot Light because of its rarity.

"It's much older, and the lighthouse is one of a kind," said Thiesen, who hopes that after the exploration the wreckage site will be included in the National Register of Historic Places .

In addition to the Coast Guard, the expedition also includes underwater archeologists from the University of Massachusetts, divers from the Massachusetts Environmental Police, and staff from the National Oceanic and Atmospheric Administration.

Not knowing the location of the lighthouse's remains, Thiesen's group selected the spot they would explore with side-scan sonar and a magnetometer to detect shapes and metal underwater. They settled on a 70-by-250 foot swath marked by buoys.

Yesterday, divers armed with plastic notebooks and mechanical pencils set out in pairs to canvass sections of the sea floor, where they found pieces of pipe and electrical cable. It wasn't clear whether the pipe was part of the old lighthouse .

Others on the Abbie Burgess peered at screens displaying underwater video shot, and transmitted, by footlong robots.

Tomorrow, the crew will anchor a floating marker with a bronze plaque at the site to commemorate Joseph Wilson and Joseph Antoine, the lightkeepers who died. "These brave men gave the last full measure of devotion to their duty to keep the light burning," the inscription on the plaque reads.

The Lighthouse Service, which once oversaw Minot Light, was incorporated into the Coast Guard in 1939 .

The new Minot Light is said to sometimes host a ghostly reminder of the past, said Jeremy D'Entremont, a lighthouse historian from Portsmouth , N.H. During storms, some mariners have seen the figure of a man reflected in the water near the new lighthouse. They say he appears to be on a ladder and that he warns them to stay away. This, they say, is the ghost of the assistant keeper who perished that night 156 years ago, after putting his note in a bottle.

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