Nemo 33 is a 33-meter deep swimming pool that is filled with specially filtered unchlorinated spring water. It has a range of different cool stuff underwater, including simulated caves, several platforms and of course the 33-meter deep cylindrical pit. In case you are wondering, that is deeper than you are allowed to go on a standard SCUBA diving license.
Video after the jump. (You might want to turn the sound down because the music is really awful)
As you might imagine, it takes a whole lot of water to fill the Nemo 33, 2,500,000 liters of the stuff to be precise, so I am guessing that it must be located in the vicinity of a natural spring. The temperature is maintained throughout the pool, which I am guessing is a good thing because I can imagine any outdoor water temperatures in and around Brussels, Belgium would be really cold. [Tech E Blog]