Off-road bicycle trails are generally function-specific and most commonly waymarked along their route. They may take the form of single routes or form part of larger complexes, known as trail centres. Off-road trails often incorporate a mix of challenging terrain, singletrack, smooth fireroads, and even paved paths. Trails with an easy or moderate technical complexity are generally deemed cross-country trails, while trails difficult even to experienced riders are more often dubbed all-mountain, freeride, or downhill. Downhilling is particularly popular at ski resorts such as Mammoth Mountain in California, USA or Whistler Blackcomb in British Columbia, Canada, where ski lifts are used to get bikes and riders to the top of the mountain.