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Olympus Mons
Tallest volcano known in the solar system
This shield volcano on Mars rises about 21 km above the Martian reference level, and up to roughly 26 km above the plains around it. That is nearly three times the height of Everest above sea level.
Olympus Mons is a shield volcano, a broad, gently sloping cone built up by countless runny lava flows rather than explosive eruptions. Its base spans roughly 600 km, which is wider than many countries, so the average slope is only a few degrees. Standing on its flank, a visitor would struggle to see that they were climbing a mountain at all.
Mars has no sea level, so scientists define height against a chosen reference. Laser altimeter data from orbit gave a summit near 21 to 22 km above the Martian datum, while the surrounding plains sit lower, which is why the gap above local terrain is quoted as larger. The summit is crowned by a complex caldera, a collapsed crater formed as magma chambers emptied beneath it.
Why so tall? Mars has lower gravity and, unlike Earth, no moving tectonic plates. A hot spot could feed the same vent for a very long time, so lava kept piling up in one place instead of being carried away. Dating by crater counts suggests the volcano is ancient, though some flows may be geologically young.