Olympus Mons
The biggest known volcano in the solar system is Olympus Mons on Mars, discovered by Mariner 9 in 1971. The mountain towers nearly 22 km (14 miles) above the surrounding plain, more than three times Mount Everest's elevation above sea level. It is more than 600 km across and occupies an area similar in size to Arizona. The cliff around the outer edge of the volcano is 6 km (3.7 miles) high. Olympus Mons is a shield volcano, similar to the much smaller Hawaiian Islands on Earth.
The image above is a mosaic from images taken by the Viking 1 orbiter in 1978. Below is a computer-generated image of Olympus Mons based on thousands of laser-altimeter elevation measurements made by the Mars Global Surveyor spacecraft.
Images: Top: NASA/JPL. Bottom: NASA/MOLA Science Team/ O. de Goursac, Adrian Lark