Jacob's Well is a perennial karstic spring in the Texas Hill Country flowing from the bed of Cypress Creek, located northwest of Wimberley, Texas. The 12-foot (3.7 m) diameter mouth of the spring serves as a popular swimming spot for the local land owners whose properties adjoin Cypress Creek. From the opening in the creek bed, Jacob's Well cave descends vertically for about 30 feet (9.1 m), then continues downward at an angle through a series of silted chambers separated by narrow restrictions, finally reaching a depth of 120 feet (37 m). Until the modern era, the Trinity Aquifer-fed natural artesian spring gushed water from the mouth of the cave, with a measured flow in 1924 of 170 US gallons per second (640 L/s), discharging 6 feet (1.8 m) into the air. The spring is the greatest source of water recharging the Edwards Aquifer.
Due to development in the area, the level of the Trinity Aquifer has dropped affecting the flow of water through Jacob's Well. In the modern era, what remains visible of the spring is a faint ripple on the surface of Cypress Creek. The spring ceased flowing for the first time in recorded history in 2000, again ceasing to flow in 2008. This resulted in now ongoing measures to address local water conservation and quality. Hays County purchased 50 acres (20 ha) of land around Jacob's Well in 2010, in an attempt to protect the spring from development. An additional thirty-one acres was transferred to the county from the neighboring Jacob’s Well Natural Area (administered by the Wimberley Valley Watershed Association (WVWA)), the new, eighty-acre (32 hectares) named the Westridge Tract.
The cave system has been explored and mapped by cave divers and has been shown to consist of two principal conduits. One passageway measures approximately 4,300 feet (1,300 m) from the surface and a secondary one extends approximately 1,000 feet (300 m) in length from the point where it diverges from the main conduit. The cave is also an attraction for open-water divers inexperienced with the specialist techniques and equipment used in cave diving, and nine fatalities have occurred at the site.
source : Wikipedia