Scales’s enthusiasm for her subject is matched by a gift for visual evocation, often disgusting. (“Picture,” she commands the reader, “a close relative of the woodlouse that hides under rocks or garden pots, but pale pink and the size of a rugby ball.” I’d rather not.) She is very good on the sublimely mountainous topography of the seabed, and the unimaginable forces that operate on its denizens. Once you are at a depth of 8km, she explains, where snailfish can live, the pressure exerted by all that seawater above is equivalent to “an elephant standing on every few square centimetres of your body”. Down there too we find scavenging crustaceans called amphipods: “They are supremely unfussy eaters and will devour anything that falls into a trench.” Like Michael Gove, they are hearty trenchermen indeed.