Argentinian artist Leandro Erlich created this illusion of people walking underwater called The Swimming Pool, for The Century Museum of Contemporary Art in Kanazawa, Japan. It's a piece of glass with water on top of it, and empty space on the bottom.
Leandro Erlich “Swimming Pool” - When seen from the deck, the pool appears to be filled with deep, shimmering water. In fact, however, a layer of water only some 10 centimeters deep is suspended over.
Swimming Pool by Leandro Erlich. Fully clothed people walk and breathe beneath the surface of a full sized swimming pool. A large, continuous piece of acrylic spans the pool and suspends water above it, creating the illusion of a standard swimming pool.