Nemo 33 is an indoor non-chlorinated fresh water facility in Brussels, Belgium. It held the record as the deepest indoor swimming pool in the world between its opening on 1 May 2004, and the completion of Y-40 in Montegrotto Terme, Padua, Italy on 5 June 2014. The facility contains a restaurant, bookshop, swimwear store, souvenir store, and rooms for other water activities. There are numerous underwater windows that allow outside visitors to look into the pools at various depths. It also offers tours around the city of Brussels.

The pool’s maximum depth is 34.5 metres (113 ft). It contains 2,500,000 litres (550,000 imp gal; 660,000 US gal) of non-chlorinated, highly filtered spring water, maintained at 30 °C (86 °F) by a solar heater, and holds several simulated underwater caves at the 10 metres (33 ft) depth level. Due to the warm temperature in the pool, divers can dive for extended periods without a dry suit. The complex was designed by Belgian diving expert John Beernaerts as a multipurpose diving instruction, recreational, and film production facility in 2004. Popular Mechanics rates Nemo 33 as one of the top 18 strangest pools in the world.

The facility allows tourists, amateur divers, and professional divers. It requires that divers be at least 12 years of age and in good health. All divers must be either certified or supervised by a trainer. All divers must have a certified diver as a dive buddy.