Explored in 2009, Vietnam's Son Doong cave has the largest known passages, in places more than 200 m tall and 150 m wide, enough to fit a 40-storey building. Where the ceiling has collapsed, sunlight streams in and a forest has grown underground. The cave is so vast it has its own microclimate, complete with clouds.
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.