Nemo33 , Brussels
The pool itself consists of a submerged structure with flat platforms at various depth levels. The pool has two large flat-bottomed areas at depth levels of 5 m and 10 m , and a large circular pit descending to a depth of 33 m . It is filled with 2,500,000 litres of non-chlorinated, highly filtered spring water maintained at 30 °C and contains several simulated underwater caves at the 10 m depth level. There are numerous underwater windows that allow outside visitors to look into the pools at various depths. 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.
Photos and full article at http://www.evilreligion.com/deepest-swimming-pool-world-nemo33-brussels/