Nemo 33 is the deepest indoor swimming pool in the world. The pool is located in Brussels, Belgium with a maximum depth is 34.5 meters (113 ft). It contains 2,500,000 liters (660,000 US gal) of non-chlorinated, highly filtered spring water maintained at 30 °C (86 °F) and holds several simulated underwater caves at the 10 meters (33 ft) depth level. Due to the warm temperature in the pool, divers can dive for extended periods without a wet suit. There are numerous underwater windows that allow outside visitors to look into the pools at various depths. The complex was designed by Belgian diving expert John Beernaerts as a multi-purpose diving instruction, recreational, and film production facility in 2004.