The deep sea, Jeffrey Marlow informs us in this fascinating book, comprises more than 95 per cent of Earth's habitable volume. It is a realm devoid of light. The sun, once thought essential for life, never penetrates it.
'For most of the denizens of the deep sea,' Marlow writes, 'light is an ungraspable, irrelevant concept.' Indeed, the deep sea was long thought to be a wasteland. No life could exist there. In the 19th century, the so-called 'azoic hypothesis' assumed that there was zero chance that any animals could survive 550m beneath the waves.
Exploration of the deep sea began with the 1870s HMS Challenger expedition that circumnavigated the globe, taking readings of the depth of the ocean floor and collecting specimens.
More than 4,000 species previously unknown to science were catalogued.
However, it was only in the second half of the 20th century that we began to understand the true biological diversity of the ocean depths. As Marlow points out, it is home to 'the vast majority of organisms on Earth'. He describes a 1977 expedition to the waters near the Galapagos. Diving in a submersible, one researcher radioed up to the ship in surprise. 'Isn't the deep ocean supposed to be a desert? There's all these animals down here.'
Life, it seemed, could thrive in the most unlikely environments: in hydrothermal vents, a kind of seafloor hot spring; in methane seeps, areas of the ocean floor where methane escapes from beneath the Earth's crust; and in the so-called Lost City Hydrothermal Field, discovered in the North Atlantic in 2000. There, spires of white rock can be seen and, in Marlow's words, 'towers seemed to melt upward, like an upside-down candle'.
Some of the creatures to be found in these assorted underwater habitats are very strange indeed. As one researcher told Marlow, 'There are so many animals that look like something you couldn't even dream up.'
There is osedax, a species of bone-eating worm in which the male is thousands of times smaller than the female; and a type of crustacean with pads on its legs 'like translucent ping-pong paddles' that it uses to propel itself through the water.
Unfortunately, no sooner have we learned to navigate the deep sea than we have started to pollute it.
On a dive to the deepest known point in the Earth's seafloor - in the Pacific's Mariana Tench - the undersea explorer Victor Vescovo was enthralled by the life he found there at 10,928m, and appalled to also see a plastic bag and sweet wrappers.
'You go to extraordinary lengths to go to these incredibly remote places,' he tells Marlow, 'and there you see human contamination.'
A deep sea explorer himself, Jeffrey Marlow understands both the delight and the horror Vescovo experienced in his dive. His book reveals the possibilities, both good and bad, that beckon as we engage more and more with the ocean depths.