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Underground aquifers, dozens of miles deep and hundreds of miles wide, are the Earth's second-largest reserve of fresh water, after ice caps and glaciers. Filled over billions of years, they are today being drained at two to four times their natural recharge rate. In this photo, a team of recreational spelunkers drop into the 160-foot deep Neversink Pit in Alabama, which local cavers have bought to preserve for future generations.

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