Located in Brussels, Belgium, Nemo 33 is the deepest indoor swimming pool in the world. Designed for divers, Nemo 33 was first conceived by John Beernaerts in 1996. The Nemo 33 diving pool has a depth of 33 meters (108 ft) at its deepest point and contains around 2.5 million liters (550,000 gal, 4.4 million pints, 660,000 US gallons) of water. The facility, which opened in June 2004, is used for dive training, leisure diving, and research. The pool also comes equipped with underwater ‘caves’ for exploration and three air-filled diving bells that enable diving instructors to communicate with their charges without returning to the surface.