The submarine canyon off California that is deeper than the Grand Canyon is Monterey Canyon.
Also called Monterey Submarine Canyon, it begins near Moss Landing in Monterey Bay and cuts across the continental shelf before extending approximately 153 kilometres beneath the Pacific Ocean. Its downstream mouth reaches depths of about 3,600 metres below sea level, while its steep walls rise roughly 1.6 kilometres in places—comparable to the depth of the Grand Canyon.
The canyon was shaped largely by turbidity currents: dense, sediment-laden flows that rush downslope underwater and erode the seafloor. Scientists still study why Monterey Canyon became so exceptionally long and deep despite being associated with the relatively modest Salinas River rather than a huge river such as the Mississippi or Amazon.
Monterey Canyon is part of a larger canyon system that also includes Soquel and Carmel Canyons. Its depth and regular delivery of nutrient-rich sediment create habitats for diverse marine life. Congo Canyon, Bering Canyon, and Hudson Canyon are real submarine canyons, but none is the named feature in this California question.