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.