Coral Reefs
I took this photo from underneath a famous, abandoned oil rig in Mancora, Peru in December 2015, from around 20 meters below the surface (max depth 38 meters, which I achieved). It is a biodiversity hotspot for providing a lot of habitat, which makes it a tourism draw for divers. On the supports of the rig are varieties of molluscs, corals and sponges which makes this an artificial reef. While here, you have the opportuntity for swimming around with wild sea lions and many species of fish.