Forests get the credit, but scientists estimate that about half of the oxygen produced on Earth comes from the ocean. It's made by phytoplankton, microscopic organisms photosynthesizing near the surface. One of them, a bacterium called Prochlorococcus, is so abundant that it alone may produce up to 20% of the oxygen in our biosphere.
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.