From WikiPedia:

Nemo 33 is a recreational diving center in Brussels, Belgium that is home to
the world's deepest swimming pool. 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 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) and contains several simulated underwater caves at the 10m 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.

You can take only a few items of your own dive kit (computer, mask, pool fins) but no BCD, fins that require booties, regulators(unless they have an integrated air linked to your computer - but no DIN fittings at all). torches, etc... You are not allowed to practice any skills unless you have an instructor in your group.