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A snowmobile, also known as a snowmachine, motor sled, motor sledge, skimobile, or snow scooter, is a motorized vehicle designed for winter travel and recreation on snow. Their engines normally drive a continuous track at the rear. Skis at the front provide directional control. The earliest snowmobiles were powered by readily available industrial four-stroke, air-cooled engines. These would quickly be replaced by lighter and more powerful two-stroke gasoline internal combustion engines and since the mid-2000s four-stroke engines had re-entered the market. The challenges of cross-country transportation in the winter led to the invention of an all-terrain vehicle specifically designed for travel across deep snow where other vehicles foundered. As of 2003, the snowmobile market has been shared between the four large North American makers and some specialized makers like the Quebec-based AD Boivin, manufacturer of the Snow Hawk and the European Alpina snowmobile. The second half of the 20th century saw the rise of recreational snowmobiling, whose riders are called snowmobilers, sledders, or slednecks. Recreational riding is known as snowcross/racing, trail riding, freestyle, boondocking, ditchbanging and grass drags. In the summertime snowmobilers can drag race on grass, asphalt strips, or even across water. Snowmobiles are sometimes modified to compete in long-distance off-road races.