The trouble with the deep sea is that it is dark and very far away. This is not the realm of glorious pink coral reefs surrounded by bewitching tropical fish. All of that lives in the light. In the depths – and in this book Jeffrey Marlow, a specialist in deep-sea microbes, goes very deep – things are different. For a start, many of the life forms which reside in the depths are indeed microbes and, in Marlow’s words, ‘all pretty much roundish blobs, even under a microscope’.

How deep is deep? We open with Marlow in a submersible called Alvin, descending to the Mid-Cayman Spreading Centre, nearly six kilometres down. This isn’t the deepest part of the ocean; that is the Challenger Deep in the western Pacific, nearly eleven kilometres below the surface. And even that is paltry compared to the oceans on some moons: Titan’s ocean is at least eighty kilometres deep. Marlow later mentions that Alvin – reconstructed several times – was first built sixty-two years ago. I wouldn’t drive a car that old, never mind descend several kilometres into the ocean. But I’m glad Marlow did, because this is a world that is rarely seen – one of methane seeps, hydrothermal vents and peanut worms. The deep sea is compelling because, despite being like nowhere else on earth, it is still a realm full of life. 

Until the middle of the 19th century, this seemed an impossibility. Whatever was down there, surely the environment was too dark, too far away, too hostile for it to contain life? At the time it was thought that water’s density increased with depth, which led to the unsettling notion that