The largest mountain/volcano in our solar system is Olympus Mons located on Mars.
Just as the largest mountain in the solar system is on Mars, so too is the deepest canyon.
Melas Chasma on Mars is 9 kilometers deep in some places, or nearly six times deeper than the Grand Canyon on average. How the giant feature was formed, itself one small stretch of the 4000-kilometer-long Valles Marineris rift valley, remains a mystery. But one thing seems evident: water played a role in its history.