In the shadows of the ocean, between the surface and a depth of 200m, unsuspected hotspots of biodiversity develop: marine animal forests. Light is scarce and there is little or no plant life: these environments are dominated by animals, from deep-sea corals to sea sponges, which grow very slowly, even more so in the polar regions. In these highly constrained living conditions, evolution has driven these organisms to adopt innovative strategies and produce unique molecules to feed, reproduce, defend themselves and so on. This biodiversity is an exceptional library in which the anti-cancer molecules, anti-virals and green chemistry of tomorrow already reside. This library is immense: it is estimated that 80% of the rocky seabed in the penumbra is covered by marine sponge assemblages. But it’s also under threat, locally from trawling and globally from climate change.