Kauai's diverse coastal reef formations provide a broad range of opportunities for adventure travelers wishing to explore this underwater world. Makua Beach ('Tunnels') on the North Shore, and Koloa Lapding and Prince Kuhio Park at Poipu Beach, are optimal locations for either snorkeling or shore diving. Poipu boasts two of Kauai's best cave dives. Sheraton Caverns are underwater grottos that range from 35 to 60 feet in depth. General Store is a 65- to 80-foot-deep reef with a wide variety of marine life and a nineteenth century steamship. At Koloa Landing, divers will discover bottles and fittings from old whaling ships and parts of the train track that once ran between Koloa and the area's sugar mill. On the North Shore, there are dramatic underwater lava tubes and archways, while on the eastern shore divers can explore the wreck of the Lukenbach, a German freighter that sank 40 years ago. Cave dives are recommended for experienced divers only. Consult the local dive shops for safety information. Guided snorkel trips are available, while outlets for do-it-yourselfers to rent snorkel equipment are accessible throughout the island. Kauai has several dive shops that offer a full range of services including two tank boat dives, introductory dives, certification courses, shore dives, night dives and nitrox dives. In addition, Ni'ihau, with its 130-foot deep reef walls and abundant marine life, is considered by many to offer some of the best diving in Hawaii.