The Marianas Trench is the deepest part of the ocean, but it is not a canyon. It is a deep-sea trench, formed by the subduction of the Pacific Plate under the Philippine Sea Plate.

While it's true that the Marianas Trench is a remarkably deep feature, it's not a canyon in the traditional sense. Canyons are typically carved by erosion, usually by rivers or glaciers. The Marianas Trench was formed by the movement of tectonic plates, a completely different process.

If you're interested in the largest canyon in the world, that would be the Grand Canyon, located in Arizona, USA.