Trail running or skyrunning is a variant on running that iffers markedly from road running and track running. Trail running generally takes place on hiking trails, most commonly single track trails, although fire roads are not uncommon. A distinguishing characteristic of the trails is that they are often inaccessible by road except at the trail heads. The trails tend to traverse varying terrain; hills, mountains, deserts, forests, and narrow passages are common. Likewise, steep inclines or rough terrain sometimes may require hiking or scrambling. Runners participating in trail runs must often descend these same steep grades. It is not atypical for trail runs to ascend and descend thousands of feet.