Nemo 33 is the deepest indoor swimming pool in the world, located in Brussels, Belgium. Its maximum depth is 113 ft. It contains highly filtered spring water and holds several simulated underwater caves at the 10 metres ft) depth level. This is AWESOME!
Going Underground: 10 Caves and Caverns to Explore in America
10 American Monuments You Need to See in Your Lifetime - Condé Nast Traveler
Cahokia Mounds State Historic Site and Cahokia Mounds Museum Society, Collinsville, IL
100 Places to Take your family: El Yunque
Jacob's Well -- Wimberly, Tx: One of the most significant natural geologic treasures in the Texas Hill Country. It is one of the longest underwater caves in Texas and an artesian spring. Jacob’s Well surges up thousands of gallons of water per minute!