Olympus (lat. Olympus Mons) - an extinct volcano on Mars, the tallest mountain in the solar system. Before the flight spacecraft (which showed that the Olympus - the mountain), this place was known to astronomers as Nix Olympica («Snows of Olympus" - due to a higher albedo).

The height of Mount Olympus - 27 miles to its base and 25 km to the average level of the surface of Mars. This is several times higher than the highest mountains on Earth. Olympus extends for 540 km wide and has steep slopes at the edges up to 7 km. The reasons for the formation of these giant cliffs have not yet found a convincing explanation.
The length of the volcanic caldera of Olympus - 85 km, width - 60 km. The depth of the caldera reaches 3 km due to the presence of six volcanic craters. For comparison - the largest in the world with Mauna Loa in Hawaii, the diameter of the crater is 6.5 km.
The atmospheric pressure at the top of Mount Olympus is only 2% of the pressure characteristic of the average Martian surface (for comparison - the pressure on the summit of Everest is 25% of that at sea level).
Olympus is a large area so that it can not be seen entirely from the planet's surface (the distance required for viewing the volcano, is so great that it will be hidden by the curvature of the surface). Therefore, a complete profile of Olympus can be seen only from the air or the orbit. Similarly, if you stand on the highest point of the volcano, its slope goes beyond the horizon.