Grand Canyon Rim-to-Rim
Grand Canyon Rim-to-Rim (R2R) is the classic one-way traverse in Grand Canyon National Park that goes from the North Kaibab Trail on the North Rim to the Bright Angel Trail on the South Rim, passing the Colorado River on suspension bridges near the bottom of the canyon.
Plan it as a very hard, long day or a multi-day hike: the rim-to-rim crossing is about 21–24 miles depending on direction/logistics, with total elevation change over 10,000 ft and long stretches where you’re descending before you eventually face the steep climb out.
Most people do North-to-South because the North Rim sits higher, which reduces the amount of climbing compared with the reverse direction; the route drops you from the Kaibab side down into the canyon interior and then brings you back out via Bright Angel.
The crossing funnels through a set of well-used canyon corridors and corridor trails: North Kaibab Trail, then the River corridor, then Bright Angel Trail. Expect a mix of steep switchback climbing/descending on the rim sections and a longer lower-elevation traverse component near the canyon bottom.
Because of extreme heat and limited water reliability in the interior, timing is critical. Canyon-floor temperatures in warm months can exceed 110°F, and many experienced hikers avoid attempting a same-day crossing during summer conditions.
For overnight options below the rim, you’re generally looking at Phantom Ranch and adjacent/nearby backcountry camping areas; the most common approach is to secure lodging/campsites well ahead and build your itinerary around where you can legally sleep.
Permits depend on whether you sleep below the rim: day-hike attempts don’t require a backcountry permit, but any overnight in the canyon requires one, managed through the park backcountry process. If you’re doing it as a long single-day push, you still need to arrange point-to-point transportation between rims.
More information: Wikipedia, Rim to Rim: Hiking across the Grand Canyon in one day, Grand Canyon Rim-To-Rim Hike - National Park Foundation