When the rising tide funnels into a narrow, shallow estuary, it can form a tidal bore: a wave, sometimes a true wall of water, that travels up the river against the current. The most impressive ones, such as on the Amazon or China's Qiantang River, can be several metres high. Some surfers ride them for kilometres.
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.