Relax and play! [Oase No. 7 by the group Haus-Rucker-Co, which was created for Documenta 5 in Kassel, Germany. An inflatable structure emerged from the façade of an existing building creating a space for relaxation and play]
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.