Where is the largest volcano in the Solar System?

Olympus Mons on Mars is about 600 kilometres across and rises some 22 kilometres - two and a half times the height of Everest, with a footprint roughly the size of France or Arizona. Its edge is a cliff several kilometres high, and it is so wide and so gently sloped that standing on it you would not perceive a mountain at all; the summit would be below the horizon.

Mars can build them because it has no plate tectonics. On Earth the crust slides over the hot spots underneath, so a plume produces a chain of volcanoes - the Hawaiian islands are one hot spot and a moving plate. On Mars nothing moves, so eruption after eruption piles onto the same spot for hundreds of millions of years. Lower gravity helps too: the mountain can get taller before its own weight starts to spread it.

It may not be finished. Crater counting on the flanks suggests lava flows only a few million years old, which is recent enough that the volcano is probably dormant rather than dead. The largest volcano in the Solar System has not necessarily erupted for the last time.