Nemo 33, Brussels, Belgium. The world's deepest swimming pool. The pool has two large flat-bottomed areas at depth levels of 5m (16 ft) and 10m (32 ft), and a large circular pit descending to a depth of 33m (108 ft). It is filled with 2,500,000 litres of non-chlorinated, highly filtered spring water maintained at 30°C (86°F). The complex was designed by Belgian diving expert John Beernaerts as a multi-purpose diving instruction, recreational, and film production facility, and opened in 2004.