Nemo 33 can be found in Belgium (Brussels to be more exact) and is the world's deepest swimming pool. It's filled with 2,500,000 litres of spring water heated to 30°C which offer complete visibility down to 33 metres in it's deepest dive pit. It has also features caves at -10 metres (including three with air pockets that are constantly refreshed) and multiple viewing windows for visitors to observe the pool from outside.

For more information and photos check out the official Nemo 33 website.