programme. Surrounded by mountains clad with native forest which has never been logged, Waikaremoana is regarded as the North Island's most attractive lake. Many native bird-species scarce in most other parts of the North Island occur in the area. A possum-hunting programme operates in the area to help protect the forest. Numerous understory species grow within the forested area of the catchment basin, crown fern (Lomaria discolor), for example. Since at least the early 1900s lake-bottom molluscs have been studied by Colenso (1811-1899) and others. The climate of Lake Waikaremoana is temperate during the summer months and cool in the winter where snow events are not unheard of. Heavy rains affect the region, especially about late winter and early spring. The weather in the area is very changeable, trampers in the region need to be on the look out for dangerous weather conditions. Hydroelectric power scheme The Waikaremoana Hydroelectric Power Scheme is a rare example of a hydroelectric power station being built on a natural landslide dam. Another example is the Tortum Dam in Turkey. Modifying the natural dam The stability of the natural dam has been the subject of intense engineering review, both at the time of construction and subsequently. Construction of an outlet tunnel through the slip, which commenced in 1935, required extensive grouting around the control structures and throughout tunnel construction. Work was suspended at the end of 1936 because Bob Semple, the newly elected Minister for Public Works, wanted the tunnelling project reconsidered for "risk, cost and value". A new tunnelling scheme was devised in 1941 based on what had been learned from initial exploratory tunnelling and work recommenced in 1943 and continued for about 5 years because of continual problems with dewatering the tunnels. After the tunnels and intake headworks had been completed the natural dam was sealed for leaks on the lake side by removing submerged timber, a task that took a year and then applying 40,000 cubic metres of crushed rock and clay-like pumice in 6 layers, then covering those layers with a top layer of larger rock and spalls to protect the material from wave action. This sealing reduced the natural flow by about 80%. The sealing of the lake was only done after tunnelling was completed otherwise it would have caused the lake level to rise and make tunnelling more difficult. As it was, the lake level had to be lowered