The Navy's Indoor Ocean

It's 240 feet wide, 360 feet long, and holds over twelve million gallon of water. This is the MASK, the maneuvering and seakeeping basin at the Naval Surface Warfare Center at Carderock.
It's one of the largest indoor oceans in the world, making the MASK the most advanced test facility of its kind.
So why does the Navy need an indoor ocean? The truth lies in these dimly-lit, somewhat mysterious waters.