I won't even try to explain this in my own words, so I'm quoting from their homepage:
andA group of divers in the Chicago area have gotten together and decided to make a way to bring tropical diving to the Midwest year-round. We will build an indoor salt water reef targeted for use by scuba divers. Scubaquarium indoor reef dive facilities will create an indoor dive experience that is unmatched in the world. The facility will provide a living reef containing coral, invertebrates, and reef fish.
That's around 800,000 gallons (look at the pool plan to see average depth)The reef is an indoor salt-water tank that is 82 feet long, 75 feet wide, and up to 20 feet deep.