Olympus Mons, meaning Mount Olympus in Latin, is the largest known mountain and volcano in the solar system. Located on Mars, it is 15.5 miles (25 km) tall, more than three times the height of Mount Everest. The mountain is actually a shield volcano, which is a large volcano with shallowly-sloping sides formed through the eruption of lava with low viscosity. Because Mars has no active plate tectonics, this hotspot used to erupt continuously in the same place, slowly building up a bigger and bigger mountain until it became the tallest and widest in the solar system.
Being a shield volcano, Olympus Mons is much wider than it is tall. The mountain is 387.7 miles (624 km) wide, topped by a caldera complex that is 49.7 miles (80 km) by 37.2 miles (60 km) wide, and up to 1.9 miles (3.2 km) deep. At the top, the air density is only 5 to 8% of that at the surface, contrasted with the top of Mount Everest, which experiences about 32% of the air density as found at the Earth's surface.
Because Mars's atmospheric density is only about 1% of the Earth's to start with, the air at the summit is only about 0.05% that of the Earth's — a veritable vacuum. Mars has lower gravity than Earth, so it also has a taller atmosphere, putting the summit of this mountain well within Mars' atmosphere.
If Olympus Mons were on Earth, a person standing on its summit could see 400 miles (643.7 km) to the horizon, in comparison to only about 3 miles (4.8 km) for observers at sea level. On Earth, 15.5 miles (25 km) is about a quarter of the way to the internationally accepted definition of space, 62.1 miles (100 km) above the planet's surface.
This volcano is often compared to Mauna Kea, in Hawaii, because both extremely large shield volcanoes by the standard of their own planet. In fact, Mauna Kea is even taller than Mount Everest, when measured from where it begins on the ocean floor.
It is extremely unlikely any mountains larger than Olympus Mons will be discovered on the rocky planets in the solar system. The discovery of such structures on the rocky cores of gas giants is also somewhat unlikely, because the greater mass of these planets would have a tendency to make them more spherical and thereby less mountainous.