In 2014, researchers described in Nature a tiny diamond brought up from the depths that held a speck of ringwoodite, a mineral that forms 410 to 660 km below the surface. The speck contained about 1.5% water, built into its crystal structure. If that proportion is typical, this layer of the mantle could store as much water as all the oceans combined, or even more.
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.