Check out these amazing underwater sculptures by artist Jason de Caires Taylor. He's created several sculptures of people/things and put them at the bottom of the ocean, where water and barnacles and natural decomposition have their way with the art. Really engaging, really eerie, really awesome.
A description from his website:
"Jason de Caires Taylor’s underwater sculptures create a unique, absorbing and expansive visual seascape. Highlighting natural ecological processes Taylor’s interventions explore the intricate relationships that exist between art and environment. His works become artificial reefs, attracting marine life, while offering the viewer privileged temporal encounters, as the shifting sand of the ocean floor, and the works change from moment to moment."
I love stuff like this. Anything that takes art into the real world, utilizing what's around us--that gets me every time. Here are just a few favorites: