The people who own the land call it "the narrows." It's a stretch of the Blanco river that cuts about 20 ft. straight down into the hillside. Maybe somebody well-versed in geology could explain why, but nowhere else in Texas have I seen a formation like this. You just walk up to it and suddenly there's this huge.. cavern or crevace or whatever you'd call it, with vegetation inside (fish too). As you walk and swim down through it, you go through a series of interconnected pools. You can't really tell from the pictures, but some of the pools are about 20-30 feet deep themselves. Water level is important too, so you can only go within a window of maybe a month or so a year that it's been raining enough to be able to swim. Anyway, it's about 20 miles from any hint of civilization, and the only way to get there is to know the owners, which is unfortunate, but it keeps it beautiful, which is worth it.
Update: Well, hey, is there anything googlemaps can't find?
Update 2: The Handbook of Texas Online says it's real, and it's never steered me wrong before
NARROWS (Hays County). The Narrows is a gorge formed by a series of cliffs seventy-five feet high and a quarter mile in length along the Blanco River in southwest Hays County, just upstream from the junction of the Blanco and Little Blanco rivers (at 30°03' N, 98°17' W). The Narrows got its name from the narrow appearance of the gorge from above. The gorge widens at the water's edge; water has eroded potholes into the rocky banks and river bottom. Springs flow from the cliffs and provide moisture for various species of ferns and watercress growing along the banks. The surrounding countryside of the Texas Hill Countryqv is characterized by sloping limestone and is used for pastureland. Live oak, juniper, and mesquite grow in the shallow clay loam. The Narrows was part of the Hermann Schlameus ranch and appeared on local tourist maps from the 1930s through the 1950s, when it was a scenic park. The ranch was sold in the early 1960s, and the Narrows was closed to the public.