A threshold trail receives lower visitation than corridor trails, but will receive more than primitive trails. The National Park Service does not regularly maintain threshold trails, but reconstructs sections damaged by environmental forces, or to prevent further trail erosion. Maintenance will also be done to protect historical features along a threshold trail. Cairns are permitted, but are to be placed discriminately. The following are designated as threshold trails:  A primitive trail receives the least visitation of all trails. The National Park Service does not regularly maintain primitive trails, but reconstructs sections damaged by environmental forces only in cases where its existing condition creates a hazard. Multiple trail eradication is done to prevent accidental off-trail hiking. Cairns are permitted, but are to be placed discriminately. The following are designated as primitive trails:  A route is a footpath that does not fall under the definition of trail because it was not deliberately constructed, or contains portions of trails that have fallen into such disrepair that they can no longer be identified on a map. Routes may exist due to cross-country hiking or animal use. Due to their difficulty, routes receive the lowest visitation of all footpaths within the park. The National Park Service only maintains routes to minimize damage to nearby natural resources. The following do not fall into any of the above trail designations, and are classified as routes:  The following hiking trails exist within Grand Canyon National Park but do not venture below the rim of the canyon.