Every December, great white sharks leave the bountiful coasts of California to congregate at a small patch of the Pacific Ocean located approximately midway between Hawaii and Mexico. As the sharks have been diving there, they have been teaching scientists that the area now known as the White Shark Café is much livelier, and more nourishing, than previously thought. Meanwhile, a supposedly humdrum stretch of the deep North Atlantic seabed called the Porcupine Abyssal Plain has been revealed as adorned with rolling hills, some of them hundreds of metres in height, where seabed organisms thrive. Ten thousand kilometres south and east of the Plain, the Indian Ocean’s Saya de Malha Bank supports the largest seagrass meadow in the world, spectacular corals and coralline algae, and, in surrounding depths, pygmy whales, flying fish, and untold others. ‘Some of the most extraordinary, most biodiverse parts of our planet are on the high seas’, writes the science journalist Olive Heffernan. ‘Yet they are unknown to most.’