Plankton—often reduced to a mere word—actually produces about 50% of the oxygen we breathe and sequesters CO₂ by transporting it to the depths. Yet, this term conceals a staggering diversity of living things, ranging from invisible microbes to extraordinary organisms with radically different ways of life, reproduction, and adaptation. There is no need to explore the cosmos: true "aliens" are already floating right before our eyes. However, they are threatened by global warming, which could diminish the production of the oxygen we breathe.