When lava cools at mid-ocean ridges, its magnetic minerals line up with Earth's field at that moment, then freeze in place. Because the seafloor spreads away on both sides of the ridge, each field reversal leaves a stripe, mirrored on either side. Recognised in the early 1960s, this zebra pattern became one of the decisive proofs of seafloor spreading.
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.