The waters just west of San Francisco's Golden Gate Bridge hide a graveyard of sunken ships. By some estimates, there are 300 wrecks in the Gulf of the Farallones National Marine Sanctuary and the Golden Gate National Recreation Area alone. But only a fraction of them have been seen by scientists.

Marine archaeologists and researchers with the National Oceanic and Atmospheric Administration have set out to document those lost vessels. Over the course of a five-day survey that ended Monday, the team used a remotely operated vehicle to identify the sites of at least four wrecks.

Maritime heritage coordinator Robert Schwemmer looks at an images from an undersea robot showing a mystery tugboat that was found by the NOAA research vessel Fulmar off the California coast. Federal researchers found four sites where ships sank in the treacherous waters outside the Golden Gate in the decades following the Gold Rush. NOAA via AP

"We're looking at an area that was a funnel to the busiest and most important American port on the Pacific Coast," said James Delgado, director of Maritime Heritage for the Office of National Marine Sanctuaries. The wrecks in the Gulf of the Farallones span a huge chunk of history, from 1595 to the present. Perhaps the best-known recent example is the tanker Puerto Rican, which exploded and sank off San Francisco in 1984. [See Photos of the Sunken Ships Near San Francisco]

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One of the newly located wrecks, the SS Selja steam freighter, was a workhorse that carried goods between the Pacific Northwest and China and Japan. On Nov. 22, 1910, the 380-foot-long (116 meters) vessel sank after it collided with a steamer named Beaver off Point Reyes, California.

A volunteer who reanalyzed a cache of NOAA sonar data found another signal that was the right size and in the right location to be the clipper ship Noonday. The vessel brought men and supplies to California during and after the Gold Rush, but it hit a rock and sank in 1863.

The team also discovered one badly broken-up wreck covered in fishnets and a mostly intact tugboat where no wreck was expected to be found, Delgado said. "We have a little homework to do there," he added.

NOAA has created an online inventory of underwater footage, sonar images, historic photographs and documents related to the wrecks that have been located.

— Megan Gannon, LiveScience

This is a condensed version of a report from LiveScience. Read the full report. Follow Megan Gannon on Twitter and Google+. Follow LiveScience on Twitter, Facebook and Google+.