Fringing reefs are most typical around islands, but also may be formed along coastlines. They usually occur where the depth of the seawater is great enough that they don't dry out and shallow enough that light penetrates to their zooxanthellae. Oftentimes, the actions of these organisms may be substantial enough to build barrier reefs, which form a barrier between the shoreline and the ocean. The most famous of these is the Great Barrier Reef in Australia. Another curiosity which can be attributed to corals are atolls.
Atolls are small ring-shaped reefs, which occur in the middle of the ocean.
The processes, which form atolls, were discovered by Darwin, who correctly postulated and later confirmed that atolls were formed as volcanoes sank back into the sea. The reefs, which formed around the ring of the volcano, were able to grow as fast as the volcano sank thus creating a ring of coral, which stayed at the surface of the ocean. Because conditions at the other edges of the atoll, i.e. the side facing the ocean, are better for coral growth than along the inner edge, the ring also grows outward, thus forming the familiar ring-shape structure of the atoll. The most notable and significant structures known to our planet are the oceanic ridges and submarine trenches, the mountains and valleys of the ocean floor. A careful look at a bathymetric map of the world ocean will give you some insight into the importance of these structures.