SIERRA NEVADA MOUNTAINS (KPIX 5) — For nearly 100 years, the Mountain Tunnel has transported the water supply from the Hetch Hetchy Dam to the Bay Area. 2.7 million customers rely on the pure water that travels through the tunnel.

For the first time in history, TV cameras went inside while engineers made repairs. KPIX 5 got an exclusive first look inside, using carefully inspected and equipped Quad Vehicles to travel five miles within.

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An exclusive first look inside of the Mountain Tunnel.(CBS)

“We are, right now, 750 feet below the surface, under the town of Groveland,” said Steve Ritchie, Assistant General Manager for Water at the San Francisco Public Utilities Commission. Engineers needed to know how the old tunnel is doing.

“We were concerned that the tunnel, at some point, could collapse, or some portions could collapse, but what we found was the liner was in better shape than we thought!” explained Ritchie

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The tunnel is 19 miles long. Engineers say they are surprised that little erosion damage happened in the last century, but some areas need critical attention. Holes and cracks are being filled, repairs that are designed to last for the next hundred years.

“Every customer that we serve receives its water from this tunnel. Yes! Right here where we are standing!” said Mountain Tunnel Project Manager Dave Tsztoo.

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Two hundred million gallons–or three hundred Olympic swimming pools–flow past the tunnel each day. The project is scheduled to be completed by March.