The formation of some submarine canyons is still a mystery. Monterey Canyon is a deep submarine canyon off the coast of the U.S. state of California. It has been compared to the Grand Canyon because of its size. It is 152 kilometers (95 miles) long and 3.2 kilometers (2 miles) deep at its deepest point. Geologists still aren’t certain how Monterey Canyon was formed. One theory is that the canyon was formed by an ancient outlet of the Sacramento or Colorado Rivers. Another theory is that it was formed by tectonic activity—an earthquake splitting apart the rock with enormous force. Scientists believe the canyon was formed 25 million to 30 million years ago.