Definition:
Bay Area Rapid Transit. BART is a light rail system that transports riders in from San Francisco to outlying areas in the East Bay and on the Peninsula (south of San Francisco). BART travels through 26 cities and 43 stations in the Bay Area.
BART also travels directly to the terminals at San Francisco International Airport and to an airport shuttle (AirBART) near Oakland International Airport.
The BART transbay tube which leads from San Francisco to Berkeley, Oakland and east is as deep as 135 feet below the Bay at some points. Designing the tube, including extensive seismic studies, took six years.
Source: BART
See the Guide to Muni and BART for help in navigating San Francisco's public transit systems.