Freestyle skiing involves aerial tricks and skiing over moguls. It also includes ski cross, half-pipe, and slope style. Heli-skiing involves downhill skiing accessed via helicopter in remote, natural mountain areas. There are various competitive skiing disciplines categorized based on gate spacing and turns including slalom, giant slalom, super G, and downhill. Cross-country skiing uses the full body and is one of the most calorie-burning sports, involving skiing on unmaintained trails without lifts. Ski mountaineering combines skiing and mountaineering techniques.