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. Its doors would eventually open to the public on May 1st, 2004.

According to the Guinness World Records:

The Nemo 33 diving pool has a depth of 33 meters (108 ft) at its deepest point and contains around 2.5 million litres (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 water is kept at a constant 30 degrees Celsius (86 degrees Fahrenheit) thanks to a large array of solar panels. There are two ‘basins’ and three ‘pits’, the deepest of which is 6 m (19 ft 7 in) across and drops to 33 m (108 ft). There are underwater ‘caves’ for exploration and three air-filled diving bells that enable diving instructors to communicate with their charges without returning to the surface. [Source]

 
For information on classes available, admission costs and directions, be sure to visit the official website at: http://www.nemo33.com/