Under the seafloor, below the surface sand or silt that divers can see, are communities of microorganisms struggling to survive in tiny pore spaces between sediment grains. A few centimeters below the surface of the seafloor, oxygen is used up. Instead of oxygen, microorganisms rely on substrates strange to us to survive. Among these microorganisms, dissimilatory sulphate reducers (microorganisms that take in dissolved sulphate) occupy a large zone in the subsurface sediments. In organic-rich water such as the Chilean shelf, they are responsible for reducing more than half of the organic carbon, out-competing aerobic respiration. Organic matter (microorganisms' food) is increasingly scarce with depth in the sediment. But communities of microorganisms can survive for millions of years. How do they do it? What is it like to live as a sulphate-reducing bacterium in sediments? And how do scientists learn about their activities in sediments hundreds of meters below the seafloor? Let’s take a journey with the microorganisms to check out the very bottom of the sea!