We also explored the ruins of the Sutro Baths:
It used to look like this:
(photo courtesy http://www.historysmith.com)
Inside, it was a stunning aquatic playground. The pools were filled with ocean water through a system of pumps that worked with the tides:
(image courtesy http://www.alamedainfo.com)
The Sutro Baths opened in 1896, and closed in 1966, shortly after being sold — upkeep of the pumps and such made it a losing proposition. Shortly after closing, a suspicious fire destroyed the Baths. The site was nearly lost to developers, but in 1980 it was purchased by the National Parks Service and added to the Golden Gate National Recreation Area.