The Blue Hole, on a rocky ledge along a straight and wide section of the Paluxy River, preserves dinosaur tracks left millions of years ago when a small pack of predatory Acrocanthosaurus crossed soft mud here, most likely while hunting prey. The site lies underwater for most of the year, and its slow-moving water and protection from freeze-thaw damage have helped preserve the tracks from erosion. Long before the tracks were discovered, the Paluxy River had been a popular swimming place for local residents since the 1800s.