God help you if you have to get your PADI certification in a public pool—you're about to spend more than a few hours steeping in all sorts of fun bodily fluids. You're better off buying a plane ticket to Brussels, home of the deepest—and likely cleanest—indoor pool in the world.

Diving enthusiast John Beernaerts began developing the idea for a super-deep indoor diving facility in 2001. Taking inspiration from favorite diving locales in his home country of Bora Bora, Beernaerts spent three years examining 21 potential designs. In 2004, Nemo 33 opened as a multi-purpose instructional, recreational, and film production facility in Brussels, Belgium.

The Nemo 33 is a 2,500,000 liter multi-level swimming pool filled with chlorine-free, potable spring water heated to a cozy 86 degrees F. The pool structure flattens out at depths of 5 and 10 meters to feature simulated underwater caves. Dry visitors are afforded port holes at both these depths to explore the terrain before they dive.

However, Nemo 33's most prominent feature is its 33 meter (108 foot) deep sinkhole. It is the only place on Earth—outside of the open ocean—that permits such deep dives. As such it's used extensively in scientific testing as well as military training exercises. In fact, during the facility's seven year operation (and after nearly 200,000 dives), the only accident to occur in Nemo 33 happened when a Belgian Special Forces member punctured both his lungs after failing to observe the prescribed ascent rate. [Like Cool - Design Boom - Nemo 33 - Image: Sergey Markov]