Jacob's Well in Wimberley, Texas. It's a significant karstic spring, the largest perennial spring in the Texas Hill Country. It flows from the most extensive underwater cave in Texas. 4m diameter mouth cave, descends vertically for about 10m, 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 40m.
The River: Poisonous Venom of the Dragon, Dragonsback [fiction: "The Dragon's Back" Christian Fantasy Trilogy]