In 1795, Governor Diego de Borica granted Jose Dario Arguello a vast Spanish land concession known as Rancho de las Pulgas -- literally, the "Ranch of the Fleas." At 35,260 acres, it was the largest land grant on the peninsula and established the pattern of enormous ranchos that would define the region's landscape until American statehood in 1850. The name suggests something about the conditions. After secularization of the missions in 1834, these grants divided the peninsula into a patchwork of cattle ranches connected by dirt tracks. The hide and tallow trade was the primary economy. Today, the names survive on street signs and subdivision plats: Miramontes, Sanchez, Arguello, Palomares.

The peninsula harbors an unlikely concentration of rare species. The San Francisco garter snake, the Mission blue butterfly, and the San Bruno elfin butterfly are all endemic to San Mateo County -- found here and nowhere else on Earth. The endangered California clapper rail nests along the bay's estuarine shores in Belmont and San Mateo. Tule elk, a subspecies found only in California, were historically native to the peninsula but were hunted to extinction here by 1850. U.S. Highway 101 in Coyote Valley south of San Jose now blocks the return of Diablo Range elk to their former peninsula habitat. These species persist in fragments of habitat surrounded by one of the most expensive real estate markets in the world.

Interstate 280 runs parallel to the San Andreas Fault down the peninsula's center, a freeway threading the gap between the Santa Cruz Mountains and the bayside suburbs. The Crystal Springs Reservoir, which fills the rift valley created by the fault, provides drinking water to San Francisco. The geography is dramatic: redwood forests and coastal scrub on the ocean side, dense urbanization and bay fill on the other, and a fault line that has produced catastrophic earthquakes running between them. The peninsula is home to San Francisco International Airport, Stanford University, Google, Meta, and Oracle -- a concentration of institutional and corporate power built on ground that has never stopped moving. The commuters who drive I-280 each day are traveling along one of the most geologically active corridors in North America, though most of them are thinking about traffic.