Olympus (Latin Olympus Mons) is an extinct volcano on Mars, the highest mountain in the solar system. Before spacecraft flights (which showed that Olympus was a mountain), this place was known to astronomers as Nix Olympica ("Snow of Olympus" - due to the higher albedo).
The height of Olympus is 27 km to its base and 25 km in relation to the average level of the surface of Mars. This is several times higher than the highest mountains on Earth. Olympus is 540 km wide and has steep slopes at the edges up to 7 km high. The reasons for the formation of these giant cliffs have not yet found a convincing explanation.
The volcanic caldera of Olympus is 85 km long and 60 km wide. The caldera is 3 km deep due to the presence of six volcanic craters. For comparison - at the largest volcano on Earth, Mauna Loa in the Hawaiian Islands, the diameter of the crater is 6.5 km.
The atmospheric pressure at the summit of Olympus is only 2% of the pressure characteristic of the average level of the Martian surface (for comparison, the pressure at the summit of Mount Everest is 25% of that at sea level).
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Olympus occupies such a large area that it is impossible to see it completely from the surface of the planet (the distance required to view the volcano is so great that it will be hidden due to the curvature of the surface). Therefore, the full profile of Olympus can only be seen from the air or orbit. Similarly, if you stand at the highest point of a volcano, then its slope will go beyond the horizon.