Big Basin and Little Basin, within a few miles of each other, are on the western edge of the Red Hills. Both formed after natural groundwater flow dissolved underground layers of salt and gypsum and the ground above collapsed into the void.
Living up to its name, Big Basin is about a mile across and more than 100 feet deep. Its walls are fairly steep and its floor—bisected by U.S. Highway 283/160—is relatively flat. Geologists estimate big basin formed within the last few thousand years.
Little Basin, to the east of Big Basin, is smaller but still about 280 yards (just under three football fields) in diameter and 35 feet from rim to floor. St. Jacob's Well, in the bottom of Little Basin, is a spring-fed pool that is also a sinkhole within a sinkhole. No one has ever reported seeing the 84-foot-wide and 58-foot-deep pool dry. New depressions that continue to appear in Little Basin are a likely sign that the processes of dissolution of underground rocks and subsidence continue.
Big Basin Prairie Preserve, which encompasses Little Basin and the eastern two-thirds of Big Basin, is managed by the Kansas Department of Wildlife, Parks and Tourism. Besides Permian and Cretaceous geology, the 1,818-acre preserve features native mixed-grass prairie and a herd of bison that helps sustain the natural ecosystem. A minimally maintained vehicle trail in the preserve runs along the southern and eastern edge of Big Basin to Little Basin, and a foot path leads from the Little Basin parking area to St. Jacob's Well.