The Vanguard project is a pilot program for the full-scale Sentinel underwater habitat, seen here, which Deep hopes to complete in 2027. A typical Sentinel crew would be six people, but the modular system could be configured to support as many as several dozen, at depths as great as 200 meters. The future of human habitation in the sea is taking shape in an abandoned quarry on the border of Wales and England. There, the ocean-exploration organization Deep has embarked on a multiyear quest to ena...