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this appeared one day about a yr ago, its in the back of my property one month it wasn't there the next it was, its about 15 ft deep, ive found thousands of cave crickets lining the walls inside the dirt in the bottom is lose cold and damp even though were in a drought no rain in close to a 6 months. its only a small diameter a little less that the size of a 55 gallon barrel. im in southwest corner of Williamson county texas.