A habitat not frequented by most divers are the vertical walls that often occur and the caves that develop as patch reefs grow toward the surface and roof over as the corals grow sideways nearer the surface. The animals that live under coral overhangs on the wall are very different from those on the reef-top or sloping fore-reef. Alcyonaceans extend into the water from the wall, especially if there is a prevailing current running along the reef. Sponges are numerous, as they do not need the sunlight as corals do. A group called coralline sponges, which slowly deposit a dense skeleton of calcium carbonate, grow in the recesses of caves, where they are the dominant organisms. At least three species of these can be found in caves so narrow that one can only look into them. They grow very slowly, but eventually fill these narrow spaces and solidify the reef from the inside. In more open conditions they are only found on deep walls, but they can be found in caves in water as shallow as 60-80 feet.