Hawaii's Mauna Loa is the most massive active volcano on the planet, with about 75,000 km³ of rock, most of it hidden beneath the ocean. It is so heavy that it presses the seafloor down by about 8 km, like a bowling ball on a mattress. It erupted again as recently as 2022.
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.