Before billionaires dreamed of setting up communities on Mars, the ocean was seen as the next frontier in human habitation. Reviving this dream is Deep, a project backed by an anonymous millionaire to the tune of more than £100m that aims to establish a ‘permanent human presence’ under the sea from 2027. Guardian Seascape editor Lisa Bachelor visited the project just outside Chepstow on the Welsh border, and tells Madeleine Finlay what scientists hope to learn about the ocean by spending extended periods living underwater. Help support our independent journalism at theguardian.com/sciencepod