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Deep in the Texas Hill Country—that’s central Texas—is nature’s perfect swimming hole. Jacob’s Well is an artesian spring northwest of the town of Wimberley. It sits on a fault line of the Edwards Plateau. Bathers often rest and play on the surface, right above a 10-foot wide shaft that leads deep, deep into the earth.
In 1948, developers made it part of a resort. You can swim at the top. But daring people don scuba gear and venture into the bowels of the spring. 10 have died while doing so.
The first chamber drops about 30 feet, then angles down 55 more. This part enjoys natural sunlight from the surface. Thereafter, Louis Bond writes, Jacob’s Well gets dangerous:
The second chamber is a long funnel to 80 feet, where there is a restricted opening to the third chamber. Inside the second chamber is a false chimney, which appears to be a way out of the well but has trapped at least one diver. Southwest Texas State University student Richard Patton lost his life in that false chimney in 1983.
The third chamber is a small room with a floor of unstable gravel. Divers must inflate water wings to navigate this chamber successfully, trying not to stir up silt or dislodge the gravel.
The passage into the fourth chamber is very tight, but the San Marcos Area Recovery Team (SMART) was recently able to penetrate it without removing their air tanks. The tightest restriction occurs 15 feet down the next tunnel where there is a knife-edge formation in the ceiling and fine gravel below. The few who have seen the fourth chamber say it is "virgin cave" with fantastic limestone formations and no gravel. Covering the bottom is fine silt that can totally obscure vision when kicked up by one misstep.
When, at great peril, divers entered the fourth chamber, they found the remains of explorers who never returned to the surface.
You can see more photos of Jacob’s Well at Amusing Planet and the Daily Mail.