Here's one swimming pool where you certainly wouldn't want to lose your locker key.
The Nemo 33 in Brussels, Belgium, is recognised by Guinness World Records as the deepest swimming pool for diving, with a staggering depth of 33 meters (108 ft).
Designed by civil engineer and diving instructor John Beernaerts, in use, it contains around 2.5 million litres (550,000 gal, 4.4 million pints, 660,000 US gallons) of water.
The facility, which opened in May 2004, is used for dive training, leisure diving, and research, and is also often used for film shoots.
The water is kept at a constant 33 °C (91.4 °F) thanks to a large array of solar panels.
There are two 'basins' and three 'pits', the deepest of which is 5m (16 ft 4 in) across and drops to 33 m (108 ft).
For swimmers that are even more adventurous, the Nemo 33 also features underwater 'caves' for exploration, while three air-filled diving bells enable diving instructors to communicate with their charges without the need for them to return to the surface.
To find out more about the pool, head to www.nemo33.com