The hills of El Dorado County are full of more than just gold; limestone caverns have also abound. Around 1900, John C. Hawver, an amateur geologist, discovered a limestone cave near Cool. The cave was explored by the University of California Berkeley in 1907 and bones of prehistoric animals and human beings were found, including the tusks from a mastodon.

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This limestone deposit provided more than just a glimpse into prehistoric life. It became a thriving commercial enterprise known as Mountain Quarries where limestone for cement and processing sugar beets was mined. At its peak production, the plant employed between 150-200 men working two eight-hour shifts and produced between 1000-1500 tons of limestone per day.

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The quarry was located on the Middle Fork of the American River in El Dorado County 7 miles east of Auburn. While limestone had been mined there on a small scale since the 1850s, the Mountain Quarries Company did not start the full-scale quarry until 1909. The quarry, a 7-mile standard gauge railroad to carry material to Auburn, and the Mountain Quarries Bridge at the confluence were built simultaneously beginning in 1910. It is estimated that over 1,000 men were employed in the construction of all three.

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Today the mine is part of Auburn State Recreation Area and is closed to the public. In the Auburn SRA General Plan, the mine has been identified as a possible interpretive area where tours may be given. The nonprofit volunteer group Auburn SRA Canyon Keepers has done extensive research on the cave and the mining operations and has been developing a plan for a guided tour program in the mine. On October 29, members of the Canyon Keepers lead a tour for district interpretive, office and law enforcement staff of the cave.

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The cave itself was formed by rainwater and snowmelt seeping into the ground and absorbing large amounts of carbon dioxide, a product of decaying organic matter. The water and carbon dioxide together form carbonic acid. This slightly acidic water dissolves the rock, forming cavities that can enlarge and join up to form a system of interconnected chambers.