With this project, called HADES, we aim to provide the first detailed, combined analysis of benthic diagenesis and microbial ecology of some of the deepest oceanic trenches on Earth. We argue that deep trenches, some of the most remote, extreme, and scantly explored habitats on Earth, are hotspots of deposition and mineralization of organic material. With the development of novel autonomous in situ instrumentation to overcome large sampling artifacts from decompression, we will i) determine rates of benthic metabolism and the importance of the deep trenches for the marine carbon and nitrogen cycles, ii) explore the unique benthic microbial communities driving these processes, and iii) investigate the proposed great role of virus in regulating microbial performance and carbon cycling in hadal sediments. By comparing trenches from contrasting oceanic settings the project provides a completely novel general analysis of hadal biogeochemistry and the role of deep trenches in the oceans, as well as fundamental new insights into the composition and functioning of microbial communities at extreme pressure.