complex crossing that spans San Francisco Bay from the city of San Francisco to Oakland via Yerba Buena Island. One of the preeminent engineering feats of the 20th century, it was built during the 1930s under the direction of C.H. Purcell. The double-deck crossing extends 8 miles (13 km) and consists of two end-to-end suspension bridges of 2,310-foot (704-metre) main spans and 1,160-foot (354-metre) side spans; an exceptionally large-bore tunnel through Yerba Buena Island that extends about 0.5 mile (about 800 m); a cantilever bridge with a main span of 1,400 feet (427 m); and a long viaduct to the Oakland shore. The greatest challenge in the construction of Bay Bridge was the sinking to bedrock (some 265 feet 【81 m】) of the central anchorage for the two suspension bridges. This feat was accomplished by use of a multiple-dome caisson invented by Daniel Moran. A section of the Bay Bridge's upper deck collapsed during an earthquake that struck the San Francisco area on Oct. 17, 1989, requiring lengthy repair.