A submarine canyon is a deeply incised, steep-sided underwater gorge or canyon that winds along or cuts across the continental shelf, sometimes continuing the line of a major river. These V-shaped canyons, whose tributaries resemble those of a river-cut land canyon, may have been formed by ﬂuctuation of sea level, uplift and faulting of landmasses, and cutting and shaping by ancient rivers. Avalanches of sand and mud in suspension scoured the canyon walls and enlarged them as sediment was carried to deep-sea basins. Off the coast of California, the numerous submarine canyons include the Mattole, Delgada, Mendocino, Carmel, Redondo, Newport, and Scripps, but none can compare in scale to the one located off the center of Monterey Bay. Monterey Canyon is as large as the Grand Canyon; it extends more than ninety miles from the coastline and plunges to a depth of thirteen thousand feet.