Viking cave is a popular dive site at phi phi island and is often visited by many dive companies throughout the day. It’s most known feature is the artificial reef, also known as the pyramids, which lie on a sandy bottom at 18 meters. The structures are made of concrete each weighing about 1000 kilos. It is wrapped by corals and sponges and shells. http://oceanslug.com/2014/08/13/artificial-reef-2/#more-583