Hydrothermal vents are places where seawater exits cracks in the sea floor, having been super-heated and enriched with metals and minerals deep in the underlying bedrock. They are an example of an ecosystem based on chemosynthesis, where life is sustained by energy from chemicals rather than energy from sunlight. The discovery of an abundance of life around deep-sea hydrothermal vents emitting hot and toxic fluids demonstrated that animals and other organisms could thrive in the dark, cold and high-pressure deep oceans. Mussels are among the most studied animals found near hydrothermal vents. Scientists discovered that mussels rely on a close, living relationship—a “symbiosis”—with bacteria for their nutrition. In this symbiosis, bacteria use chemicals from the hydrothermal fluid and seawater to produce organic compounds, while the mussels provide the bacteria with essential compounds and protection. The mussel life cycle is uniquely adapted to finding and colonizing their unusual habitat and then finding suitable symbiotic bacteria, almost immediately. Despite its remoteness, the deep sea is already under threat. Although there is still much work to be done, research into mussels and other animals that have evolved similar symbioses has revealed not only their beauty, but also their fragility.