Trail running is a sport-activity that combines running, and, where there are steep gradients, hiking, that is run "on any unpaved surface". It is similar to both mountain and fell running (also known as hill running). Mountain running may, however, include paved sections. Trail running normally takes place in warm climates, on good paths, or tracks that are relatively easy to follow, and does not necessarily involve the significant amounts of ascent, or need for navigating skills, normal in fell running. Unlike road running and track running, it generally takes place on hiking trails, often in mountainous terrain, where there can be much larger ascents and descents. It is difficult to definitively distinguish trail running from cross country running. In general, however, cross country is an IAAF-governed discipline that is typically raced over shorter distances. The number of organized trail races grew by 1,000% from 2008 to 2018, from 160 to more than 1,800 globally. Runners often cite less impact stress compared to road running, as well as the landscape and non-urban environment, as primary reasons for preferring trail running. This move to nature is also reflected in a large increase in competitors in non-traditional/off-road triathlons and adventure racing in the 2010s. Related activities Fastpacking A growing number of people are participating in solo backcountry trail running trips, carrying an ultralight form of backpacking to allow faster speeds than with a traditional backpack. Running while backpacking has been termed "fastpacking". These trips can be both difficult and dangerous, depending on length, weather, and terrain. Mountain and fell running Mountain and fell running (also called hill running, particularly in Scotland) are sports that combine running and racing off-road over the upland country, where the gradient climbed is a significant component. Fell is a dialect word from the northwest of England where it is popular–especially in the Lake District. Fell races require mountain navigation skills and participants carry survival equipment. Unlike trail running, the routes of fell races are often unmarked so that competitors frequently are able to choose their own route to a checkpoint. The only difference between mountain running and trail running is that a mountain running course sometimes includes paving. It is different from fell running because (1) courses are clearly marked and avoid dangerous sections; and (2) while mountain running takes place mainly off-road, if there is significant elevation gain on the route, surfaced roads may be used.