Montezuma Well National Monument Rimrock

Montezuma Well, a detached unit of Montezuma Castle National Monument, is a natural limestone sinkhole near the town of Lake Montezuma, Arizona, through which some 1,500,000 US gallons of water emerge each day from an underground spring. It is located about 11 miles northeast of Montezuma Castle.

The "well" measures 386 feet in diameter from rim to rim and contains a near-constant volume of spring water even in times of severe drought. The water is highly carbonated and contains high levels of arsenic. At least five endemic species, the most of any spring in the southwestern United States, are found exclusively in Montezuma Well: a diatom, the Montezuma Well springsnail, a water scorpion, the Hyalella montezuma amphipod, and the Motobdella montezuma leech.

Source: Wikipedia