We have all heard that living creatures need the sun to survive, either directly through photosynthesis or by consuming biomass produced by photosynthesis. But is this really true for all of them? Some organisms survive in the deep sea – where there is no sunlight – using energy from our planet instead of our sun in a process known as chemosynthesis. We wanted to know more about these microorganisms, especially the ones that live below the sea floor at deep-sea hot springs. So we examined fluids from a hot spring in the Pacific at in situ pressure and temperature and measured their productivity and rates of metabolism. Our study showed that sub-seafloor microorganisms were highly productive and fast-growing. Our estimates allow us to assess the importance of hot springs for global cycles of essential elements, such as carbon and nitrogen.