Fires ravaged San Francisco's Market Street after a major earthquake hit the city in 1906 (top), but no evidence of the destruction remained by 2003 (bottom).

Fire is still a major threat to San Francisco in the event of an earthquake, primarily because the city's water supply is so vulnerable.

Eighty-five percent of the city's water arrives via a single pipeline from the Sierra Nevada mountains, more than 150 miles (240 kilometers) away. On its journey to San Francisco the pipeline crosses three major earthquake faults.

Scientists estimate that a major quake today could leave the Bay City without running water for as much as two months.