Five miles long and nine feet in diameter, the Bay Tunnel runs under San Francisco Bay from Newark to Menlo Park. It is the first bored tunnel beneath the bay -- not for cars or trains but for water, carrying treated drinking water from the Hetch Hetchy system to customers on the Peninsula. Completed by the San Francisco Public Utilities Commission, the tunnel replaced an aging pipeline and demonstrated that tunneling technology could conquer the bay's challenging geology.
The tunnel was drilled using a tunnel boring machine through the bay's submarine geology -- a mix of soft sediments and harder formations that required careful engineering. At nine feet in diameter, it is large enough for maintenance crews to walk through but small enough to be efficient for its purpose: conveying water. The five-mile route passes entirely beneath the bay floor, connecting Newark on the East Bay side to Menlo Park on the Peninsula.
The existing pipeline that crossed the bay was aging and vulnerable to earthquakes -- a particular concern in a region that straddles the San Andreas and Hayward faults. The Bay Tunnel provides a seismically resilient alternative, buried deep enough to be protected from surface disruptions. It is part of the San Francisco Public Utilities Commission's broader program to modernize the Hetch Hetchy water system, which delivers Sierra Nevada snowmelt to 2.7 million Bay Area customers.
From the air, the Bay Tunnel is completely invisible -- there is no surface expression of the five miles of engineered tube running beneath the water. Its entry and exit points are modest utility structures, unremarkable to anyone who does not know what they connect. This invisibility is characteristic of the infrastructure that sustains modern cities: the most essential systems are the ones you never see.
The Bay Tunnel runs from approximately 37.485°N, 122.132°W (midpoint) beneath San Francisco Bay from Newark to Menlo Park. There is no surface expression visible from altitude. Nearest airports: Palo Alto (KPAO) 3 nm south, Hayward Executive (KHWD) 8 nm north.