The series is an Anglicisation of Valkyrien, a Norwegian thriller set in the disused Oslo metro station of that name. That history might explain the British version’s slightly baffling range of mythic references. There’s a lot of Paradise Lost sloshing about in its script. (Dr Milton has a daughter called Eve.) But the hero is more Prometheus than Satan – his founding crime is to steal an experimental cancer drug in the hope of saving his wife. And there’s also something Wagnerian going on here. Mark Strong’s cavernous base looks a great place for any Nibelung to set up his anvil.

The Tube was built to get Londoners to work, but it’s had plenty of time to develop its own system of myths and symbols. In Underground (1928), the director Anthony Asquith shot the escalators and uplighters of Waterloo as if they were the fittings of a gleaming science-fiction city. In Would You Believe It (1929), the now-forgotten comedian Walter Forde created a breathless subterranean chase sequence involving a spiral staircase, a lift and a gaggle of Russian spies. In those days, the Tube was a place to gawk at the choreography of urban bodies in motion.

The Second World War changed that – and it wasn’t part of the plan. During the Blitz, the treasures of the British Museum were given a safe berth in the tunnels, but Churchill’s cabinet was unwilling to extend this privilege to Londoners under fire. Officials believed that if the Underground became a place of refuge from the Luftwaffe, a “deep shelter mentality” would develop and people would refuse to emerge. According to the scientist J B S Haldane, some of the more paranoid members of the British establishment also worried that deep shelters might be used as revolutionary bases in the event of an uprising. The decision was reversed after activists took direct action with bolt-cutters. The stations were opened during air raids and purpose-built shelters added to eight stops on the northern and central lines.