I was walking along the Embarcadero the other day and passed this sign but then quickly doubled back to read the sign because it intrigued me. I had heard about ships that were buried under San Francisco as I was watching a documentary once about the city. I had no idea just how many were actually below the surface! After some digging around, this is what I've found:
From this website, some information about the buried ships:
During the Gold Rush of 1849 and 1850s there were no railroads, airplanes, or automobiles. The fastest mode of transportation to the first stop for the gold fields, San Francisco, was aboard a vessel. By the summer of 1850, over 500 vessels were recorded as being anchored in the vicinity of Yerba Buena Cove. After they had arrived, whole crews abandoned their ships, along with the passengers, to make their way up to the gold fields. Many of the vessels were eventually left to rot, others were eventually used for such purposes as storeships, saloons, hotels, jails, and some were sunk purposefully to secure water lot titles (property that was originally underwater). As wood was scarce at the time, due to the many fires that swept the city and the increasing need for building material, many of the vessels were also broken up for their timber as well as other parts such as the metal plating.
By 1851, the wharves had extended out into the cove and numerous buildings had been erected on piles near them. Over the next two decades, under various waterfront extension bills, Yerba Buena Cove was filled with sand from the downtown area. According to Bancroft, a local historian, "As late as Jan '57 old hulks still obstructed the harbor while others had been overtaken by the bayward march of the city front and formed basements or cellars to tenements built on their decks. Even now  remains of the vessels are found under the filled foundations of houses." The cove was eventually enclosed by a seawall which was built from 1867 to 1869, and which followed roughly along the same path as The Embarcadero.
This image below from google shows the buried treasure under SF. The green shows the original shoreline and the added land to expand the city's surface area. How freaking cool is that?!?!?!