Olympus Mons is the largest known volcano in the solar system, and the youngest large volcano to be found on Mars. Its last eruption may have been as recently as 25 million years ago.
The shield surrounding Olympus Mons has about the same area as the U.S. state of Arizona. One reason the large volcanoes on Mars are so huge, is that the crust of Mars does not move like the Earth’s crustal plates do.
On Earth, the crust moves over plumes of magma from deep in the Earth, creating a series of small volcanoes, for example the Hawaiian islands. On Mars, the crust remains stationary and volcanoes grow larger and larger over time.
Visible from Earth with telescopes, Olympus mons was first spotted in the early 1800's, poking out of the top of the global dust storms that frequently rage around Mars. The volcano was first photographed up-close by the Mariner 4 probe, which sped past Mars in 1964.