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Spotlight: Mountain Lake Tunnel
A photo of the opening to the Mountain Lake Tunnel. The muddy entry point was unearth near El Polin spring.
By the end of October, Trust archaeologists hope to find the mouth of an unfinished Mountain Lake Water Company tunnel constructed in 1853. Their goal is to search the east arm of Mountain Lake for any significant archaeological deposits before dredging of the lake begins in November.
 
What is the Mountain Lake Water Company Tunnel?
The rapid growth of San Francisco following the Gold Rush created an almost insatiable demand for water but the city lacked a ready source. Entrepreneurs saw an opportunity and proposed several competing schemes to supply water at a profit.
 
In 1851, a group of investors devised a plan to build a three-mile tunnel from Mountain Lake in the Presidio to the intersection of Pacific and Larkin in San Francisco. Thus, the Mountain Lake Water Company was born. With solid investor backing and much fanfare, the company broke ground for the tunnel in 1853.
 
As reported in the day’s newspaper, over a thousand people attended a groundbreaking ceremony to enjoy a live band and hear speeches, each anticipating a day when the completed tunnel would deliver an unending supply of water for “preventing conflagration, cleansing the streets, laying the dust, purifying air, earth and people, obviating medicine, insuring property without premium, preserving life from flames and hearts from fears.”
 
Logistical hurdles, funding shortfalls, and investor lawsuits, however, eventually forced the company into bankruptcy. Although some clues survived the years, historic records left the fate of the tunnel a mystery.
 
Recent Discovery
Archaeologists and historians long wondered how much of the tunnel was completed before the project failed and how much of the monumental structure may remain buried for exploration. Using historic maps and other clues, archaeologists zeroed in on a spot just north of El Polín Spring to begin their search. There, in October 2010, the Presidio Archaeology Lab discovered an outfall and spillway structure for the Mountain Lake Water Company Tunnel buried more than 40 feet below the ground.
 
Built directly into the serpentine bedrock, the tunnel was solid brick with a four foot tall opening and a complex set of structures meant to catch mud and silt before the water headed into a large reservoir. Unfortunately, the tunnel was buried far too deep for it to remain open for public visitation.
 
Can it be Found Again?
Although the part of the tunnel discovered near El Polín had to be reburied for safety and logistical reasons, archaeologists were interested in finding the mouth of the tunnel. During the excavation, archaeologists will dig three trenches around the east arm of the lake. After the tunnel mouth is unearthed, it will be available to the public as a teaching tool, helping visitors interpret this fascinating chapter in the history of San Francisco.
 
Will they find it? Stay tuned for updates on this research in the coming weeks.
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