There used to be a lot less city and a lot more wharf. But when dozens of ships were abandoned by gold-seeking sailors, they eventually rotted and sank, and as the city expanded out into the bay, they were buried beneath it.
(Which gives me some fun ideas for a story, by the bye.)
Ships under Financial District – FoundSF
Unfinished History. Downtwn1$yerba-buena-cove-1851.jpg. Photograph of Yerba Buena Cove in 1851. Map of ships buried under San Francisco Financial District. Map by Sam Manera, 2013, based on an earlier map by Ron Filion, 2000. Satty-p-32-asbury-quote-ships.jpg. Collage depicting ships piled into …
This is pretty much the same story as Manhattan, except substitute ships with garbage and refuse. It wouldn't surprise me if there's more than a few mafia related bodies in there as well.
http://gizmodo.com/5-parts-of-nyc-built-on-garbage-and-waste-1682267605
Yeasdani Sk