When Mariner 9 reached Mars in 1971, a global dust storm had hidden the entire planet. As the dust settled, a few peaks poked through the clouds, and scientists realized they were looking at volcanoes so tall their summits pierced a planet-wide storm. One of them, once named Nix Olympica by 19th-century astronomers, was confirmed as Olympus Mons, the largest planetary mountain in the solar system. This episode is dedicated to understanding its almost comical scale.

We cover the numbers: up to 26 kilometers high, about 600 kilometers across, a base that would cover Italy, a summit pressure of just 72 pascals, and a slope so gradual that an astronaut on its flank could not see the top or the bottom because of the planet’s curvature. We explain why Mars, without mobile tectonic plates and with a 70-kilometer-thick lithosphere and lower gravity, can build a shield volcano this size while Earth cannot. We then dig into the mountain’s lopsided shape, the compressional features on its Tharsis side, the giant landslides on its northwest flank, and the water-saturated clay sediments that may have acted as a detachment zone. Finally, we look at lava flows as young as two million years, a magma chamber roughly 32 kilometers down, and why warm underground springs could make this a top astrobiology target.

• How the 1971 dust storm delayed and then dramatically revealed the Martian volcanoes
• Why the Hawaiian Islands form a chain but Olympus Mons piled up in one spot
• The six nested summit calderas and the basal escarpment up to eight kilometers tall
• How wet phyllosilicate clays under the northwest flank may explain catastrophic collapses
• The craters Karzok and Pangboche and the shergottite meteorites thought to have been blasted to Earth