Not much is known about the sea, it is said; the surface of Mars is better mapped.
But 2,000 holes have now been drilled in the bottom, 100,000 photographs have been taken, satellites monitor the five oceans and everywhere floats fitted with instruments rise and fall like perpetual yo-yos. Quite a lot is known, and very little is reassuring.
The worries begin at the surface, where an atmosphere newly laden with man-made carbon dioxide interacts with the briny. The sea has thus become more acidic, making life difficult, if not impossible, for marine organisms with calcium-carbonate shells or skeletons.
These are not all as familiar as shrimps and lobsters, yet species like krill, tiny shrimp-like creatures, play a crucial part in the food chain: kill them off, and you may kill off their predators, whose predators may be the ones you enjoy served fried, grilled or with sauce tartare. Worse, you may destabilise an entire ecosystem.
That is also what acidification does to coral reefs, especially if they are already suffering from overfishing, overheating or pollution. Many are, and most are therefore gravely damaged. Some scientists believe that coral reefs, home to a quarter of all marine species, may virtually disappear within a few decades. That would be the end of the rainforests of the seas.
Carbon dioxide affects the sea in other ways, too, notably through global warming. The oceans expand as they warm up. They are also swollen by melting glaciers, ice caps and ice sheets: Greenland