<P> Most of the world's rivers are fed from mountain sources, with snow acting as a storage mechanism for downstream users . More than half of humanity depends on mountains for water . </P> <P> In geopolitics mountains are often seen as preferable "natural boundaries" between polities . </P> <P> Mountaineering, mountain climbing, or alpinism is the sport, hobby or profession of hiking, skiing, and climbing mountains . While mountaineering began as attempts to reach the highest point of unclimbed big mountains it has branched into specializations that address different aspects of the mountain and consists of three areas: rock - craft, snow - craft and skiing, depending on whether the route chosen is over rock, snow or ice . All require experience, athletic ability, and technical knowledge to maintain safety . </P> <P> Heights of mountains are typically measured above sea level . Using this metric, Mount Everest is the highest mountain on Earth, at 8,848 metres (29,029 ft). There are at least 100 mountains with heights of over 7,200 metres (23,622 ft) above sea level, all of which are located in central and southern Asia . The highest mountains above sea level are generally not the highest above the surrounding terrain . There is no precise definition of surrounding base, but Denali, Mount Kilimanjaro and Nanga Parbat are possible candidates for the tallest mountain on land by this measure . The bases of mountain islands are below sea level, and given this consideration Mauna Kea (4,207 m (13,802 ft) above sea level) is the world's tallest mountain and volcano, rising about 10,203 m (33,474 ft) from the Pacific Ocean floor . </P>

When does a hill become a mountain wikipedia