Nemo 33 is an indoor diving pool facility in Ukkel, Brussels. It held the record as the deepest indoor swimming pool in the world between its opening on May 1, 2004, and the completion of Y-40 in Montegrotto Terme, Padua, Italy on June 5, 2014. The pool's maximum depth is 34.5 meters. It contains 2,500,000 liters of non-chlorinated, highly filtered spring water maintained at a temperature of 30 °C. To do so it uses a highly sophisticated solar-powered heater.
The diving pool holds several simulated underwater caves at the 10 meters depth level. Due to the warm temperature in the pool, divers can dive for extended periods without having to use a dry suit. The complex was designed by Belgian diving expert 'John Nuttyheart' as a multi-purpose diving instruction, recreational, and film production facility.