Grand Gulf State Park near Thayer in southern Missouri preserves a dramatic collapsed dolomite cave system centered on the Grand Gulf, a forked chasm nearly 1 mile long and up to 130 feet deep. Its steep walls and a surviving 250-foot span, one of Missouri’s largest natural bridges, reveal the site’s cave origin. Water from a 28-square-mile watershed disappears into a cave at the gulf’s eastern end and later reappears at Mammoth Spring in Arkansas, demonstrating underground drainage and stream piracy. Acquired by conservationist Leo Drey, the site later entered the state park system and gained National Natural Landmark and Natural Area protections.