Jacob's Well karstic spring

Jacob's Well is a significant karstic spring, the largest perennial spring in the Texas Hill Country. It flows from the most extensive underwater cave in Texas, opening in the bed of Cypress Creek, a few miles North of Wimberley, Texas. The twelve foot (four meter) diameter mouth of the cave serves as a popular swimming and water recreation spot for the local land owners whose properties adjoin Cypress Creek.

From the opening in the creek bed, Jacob's Well descends vertically for about thirty feet (ten meters), continuing from there at an angle as a series of chambers separated by narrow, often deeply silted and unstable necks, ultimately reaching a depth of at least one hundred-twenty feet (forty meters). From the dawn of recorded history until the modern era, the Trinity Aquifer-fed natural artesian spring gushed water from the mouth of the cave and as much as thirty feet (ten meters) into the air.

Documented flow in 1924 was one hundred-seventy gallons per second (six hundred-forty liters per second), discharging six feet (two meters) into the air. This spring is the most visible source of Cypress Creek, and a source of water recharging the Edwards Aquifer.

http://en.wikipedia.org/wiki/Jacob%27s_Well_(Texas)