A deep valley with steep sides is a canyon. Canyons are cut by erosion, and they provide insight about the geology of the region. Rivers are the main creators of canyons on land, and torrents of water under the ocean cause underwater canyons.
Canyons can be created when a spring of water cracks through the surface of the bottom of a cliff. The water penetrates the porous rock until reaching a layer that it cannot penetrate. This forces the water to travel sideways, which eventually collapses the water's initial exit point. As the cliff continues to collapse, a box canyon is formed.
Slot canyons are formed when occasional bursts of rushing water travel through eroding plateaus. Some slot canyons can be hundreds of feet deep but only a few feet across. In the ocean, some currents crash to the ocean floor. Over time, this erodes it and causes a canyon. Other underwater canyons are caused by rivers dumping into the ocean and eroding the continental shelf.
The Grand Canyon in Arizona is the world's most famous canyon. It was created by the flowing of the Colorado River over the course of millions of years. It is 5,000 feet deep, 277 miles long and 18 miles across at its widest point.