A stealthy autonomous underwater robot that can track elusive underwater creatures without disturbing them could help us better understand the largest daily migration of life on Earth. Mesobot, a 250-kilogram robot that operates either unconnected to a power source or tethered with a lightweight fibre-optic cable, is able to move around below the surface unobtrusively. The ocean's twilight zone – known more formally as the mesopelagic zone – lies between about 200 metres and 1 kilometre in depth. It is the site of the diel vertical migration (DVM), a daily phenomenon during which deep-dwelling animals come closer to the surface to feed on the more plentiful food supplies found there, while dodging predators. The DVM is seen by biologists as a very important way in which nutrients – and carbon dioxide captured via photosynthesis – can be rapidly transported to depth, where carbon can be stored for the long term.