The largest migration of life on earth departs every night from the twilight zone, the kilometre-deep middle layer of open ocean in which the majority of living creatures can be found. As darkness falls, millions of tons of animals, ranging in size from the smallest arrow worms to the largest cetaceans, swim their way up to the photic zone to feed in relative safety, braving shallower waters under cover of night to gorge themselves on nutrients - and on one another - before plunging back into the gloomy depths as dawn begins to break. For a few short hours, the top thirty metres of the world's great oceans teem like overstocked aquaria. The process, known as vertical migration, was discovered relatively recently, and as yet scant details of its natural history have been collected by marine zoologists, for whom many of the goings-on in the ocean's deeper regions remain just as mysterious and out of reach as they were when ocean science began in earnest in the mid-19th century. 'The great depths of the ocean are entirely unknown to us,' Jules Verne declared in 1869, in the early pages of Twenty Thousand Leagues under the Sea. 'What passes in those remote depths - what beings live, or can live, twelve or fifteen miles beneath the surface of the waters - what is the organisation of these animals, we can scarcely conjecture.' Much the same could be said today, 140 years on from the voyage of the Nautilus, with less than 5 per cent of the world's 320 million cubic miles of ocean having so far been explored, and an estimated 50 million unknown species thriving in its depths.

LRB 3 November 2005 | PDF Download