The Nemo 33 diving complex in Belgium was designed by expert diver John Beernaerts as a multi-purpose diving instruction, recreational and film production space unlike any in the world. It took seven years to build and the 34.5 meter (113 feet) pool in Belgium was open for business in the summer of 2004. Solar panels heat the 2.5 million liters (660,430 gallons) of non-chlorinated, highly-filtered spring water to keep it at 30 degrees Celsius (86 degrees Fahrenheit). Within Nemo 33 there are numerous underwater caverns to help train divers and windows for dry outsiders to get a peek inside.
Image screenshot of video from loiphin on youtube