In 1894 the Spring Valley Water Company needed a place to store water that was pumped from the Laguna Honda Reservoir on the western side of the ridge of hills that runs through the center of San Francisco. Tank Hill was perfectly positioned just east of the saddle running between Twin Peaks and Mount Sutro and the Reservoir and rapidly growing downtown.
The tank was removed in the 50's, and the City wisely rejected plans for a housing development and instead purchased the land with monies from the Open Space Fund.
Nowadays it remains one of San Francisco's best-kept *secrets*. Most people miss it as they drive buy on their way to Twin Peaks. Tank Hill gives panoramic views from Ocean Beach to Point Reyes and all the way to Oakland and Bayview, with benches and a rope swing. There are two official trails to the summit, but enterprising hikers (and trail runners) know of a glorious (but short) single-track n' scramble up the northern, rugged face.