As of mid-2025, the international Seabed 2030 project estimated that about 27% of the ocean floor had been mapped with modern sonar. For the rest, we rely on far coarser satellite estimates that can miss entire seamounts. The upshot: we have sharper maps of the surface of Mars than of the bottom of our own oceans.
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.