Nemo 33 is a diving center in Belgium home to the world’s deepest swimming pool. Filled with 2,500,000 litres of non-chlorinated and highly filtered spring water, it is kept at a constant temperature of 30°C (86°F). It has three pits measuring 5m (16 ft), 10m (32 ft) and the deepest pit at a depth of 33m (108 ft).
To keep things interesting for practicing divers, there are a number of specially built underwater caves. Designed by Belgian dive expert John Beernaerts.