Crystal Springs Reservoir is actually two connected bodies of water in the northern Santa Cruz Mountains of San Mateo County. The Upper reservoir, originally a natural lake called Laguna Grande, sits two miles south of Crystal Springs Dam on Canada Road. The Lower reservoir receives water from San Mateo Creek and San Andreas Creek, plus overflow from the Upper reservoir via tunnels beneath Highway 92. In 1924, culverts were built through the dam separating them, linking both hydraulically. Part of their water comes from local precipitation. The rest arrives through one of engineering's great feats: the Hetch Hetchy Aqueduct, which pipes water 167 miles from Yosemite National Park, supplemented by the Pilarcitos Creek and Alameda Creek watersheds. The reservoirs are operated by the San Francisco Public Utilities Commission.

The Portola expedition camped at the site of what is now Upper Crystal Springs Reservoir on November 5, 1769. Fray Juan Crespi described a lake teeming with ducks and geese, surrounded by evidence of elk and deer in astonishing numbers -- scouts counted fifty deer in a single group. Three Lamchin Ohlone villagers approached the expedition bearing gifts of black pies and a kind of cherry, inviting the strangers to their village. The Lamchins were a group of roughly 350 people whose territory included present-day Redwood City and Woodside. Their villages -- Cachanigtac, Guloisnistac, Oromstac, Supichom -- cannot be precisely located today. The natural lake they knew as home now lies beneath millions of gallons of municipal water supply, marked only by a California Historical plaque.

In March 2012, a pair of bald eagles built a nest in a coast Douglas fir at the northwest corner of the lower reservoir -- the first reported bald eagle nest in San Mateo County since 1915, nearly a century earlier. The birds' initial attempt failed, but they returned the following year and successfully fledged a chick that flew north after leaving the nest. The eagles' presence is a marker of ecological recovery in a landscape shaped by infrastructure. The reservoirs sit directly atop the San Andreas Fault, the tectonic boundary that generated the devastating 1906 earthquake. Crystal Springs Dam, completed in 1888, survived that quake intact, a testament to its construction. The Crystal Springs Regional Trail now runs along the reservoir's edge, offering views of water, fault line, and the hills of Filoli estate -- a world that feels remarkably wild given its location between Silicon Valley's suburbs and San Francisco's southern edge.