During some eruptions, a mix of scorching gas, ash and rock collapses and rushes down the volcano: a pyroclastic flow. It often exceeds 100 km/h, and the most violent ones approach 700 km/h, at temperatures of several hundred degrees. Impossible to outrun, it is among the deadliest volcanic hazards, and it is what struck Pompeii.
On 27 August 1883, the eruption of the Indonesian volcano Krakatoa was heard as far away as Rodrigues Island, nearly 4,800 km away. Its pressure wave was picked up by barometers worldwide and circled the globe several times. The tsunamis that followed killed more than 36,000 people.
The deepest known point in the ocean, Challenger Deep, lies roughly 10,900 m below the Pacific surface. Drop Everest in and its summit would still sit about 2 km underwater. Down there, the pressure is more than 1,000 times what you feel at sea level, roughly a tonne pressing on every square centimetre.