case the piston pushing it would be on a hinge too), or the panel sitting on rails. This method is becoming increasingly rare, due to the high maintenance it requires. A famous example of this mechanism is World Waterpark's Blue Thunder Wave Pool Water tank. This is usually the strongest type of wave machine, utilising large water tanks behind the pool that have water pumped into them, before a valve at its base opens. This allows the water to flow through a channel that slightly angles upwards as it enters the back of the pool, creating sometimes huge waves. When this happens, the wave machine can sometimes make a loud roaring sound, as the air fills the emptying chambers, for example on Disney's Typhoon Lagoon. Famous examples include Siam Park's Wave Palace, holding the record for largest man made waves at 3.3 metres, and the previously mentioned Disney's Typhoon Lagoon. Other techniques utilize an "accordion mechanism" which opens and closes in order to suck water into its belly (opening) and push it out (closing) to cause waves. Types and locations Generally, wave pools are designed to use fresh water at inland locations, but some of the largest ones, near other seashore developments, use salt water. Wave pools are typically larger than other recreational swimming pools and for that reason are often in parks or other large, open areas. Some wave pools like those made by Wavegarden at Surf Snowdonia and NLand are expressly designed for surfing rather than for swimming, and accordingly, create much larger waves. Other surfing wave pool projects, some of which can be in lakes, include Surf Ranch from Kelly Slater Wave Company, Surf Lakes, Webber Wave Pools and Okahina Wave. Surfing can also be done on a static surf simulator but is less realistic than surfing wave pools due to the static wave. Safety Wave pools are more difficult to lifeguard than still pools as the moving water (sometimes combined with sun glare) make it difficult to watch all swimmers. Unlike passive pool safety camera systems, computer-automated drowning detection systems do not work in wave pools. There are also safety concerns in regards to water quality, as wave pools are difficult to chlorinate. The original eight-foot-deep (approx 2.4m) Tidal Wave pool at New Jersey's Action Park cost three lives in the 1980s, and kept the lifeguards busy rescuing patrons who overestimated their swimming ability. On the