The Sunset Reservoir is San Francisco's largest, and serves 60 percent of city residents and businesses. CBS 5 was able to take a rare look inside the underground reservoir on Thursday, which looks like an ancient Roman temple.

For the past two years, the Sunset Reservoir has been empty while workers strengthen the walls and columns. The work is just one of 80 projects to strengthen the Hetch Hetchy water delivery system ahead of the next major earthquake.

The repair work is a race against time for the inevitable earthquake. If the reservoir is not completed, huge parts of San Francisco and the Bay Area could be without water for up to two months following a major quake.

VIEW the segment from CBS5.com