Free solo climbing is the riskiest of all forms of climbing. There are inherent dangers to climbing solo, as a partner can help with belaying or even call for help in case of an accident. In free climbing, the person not only climbs alone, but he also climbs without any ropes or harnesses to protect him in case of a slip or fall. The most dangerous part is not the climbing itself, as most solo climbers choose routes they already know well. The major danger is the absence of help in case of emergency, plus the exposure to the environment, including the danger of falling rocks and slippery icy edges.
For it to be considered true free solo climbing, the climber must be at least 25 feet (7.62 meters) off the ground, a height from which it would be extremely unlikely that anyone would survive a fall. Even the most experienced climbers consider this "a sport for the crazy." A similar form, known as deep-water soloing, consists of climbing on sea cliffs. The presence of water down below may seem to lower the risks, but in fact increases them, as the rock tends to be slippery, and the tides and underwater boulders make a fall as deadly as one that happens away from water. Free solo climbing is considered an extreme sport and is illegal in many national parks, which is why is hard to know exactly how many people practice it.
One of the most famous climbers was Derek Hersey, who died when climbing Sentinel Rock in the Yosemite Valley in 1993. Dan Osman, sometimes called "the king of free solo climbing," died in 1998 during a "controlled free-falling," a form of bungee-jumping. Osman had spent most of his adult life climbing alone, and had even starred in several documentaries about the extreme sport. Other famous climbers include Dean Potter, who is also an expert at speed climbing, and Michael Reardon, who is famous for climbing towers and skyscrapers without ropes.