When you walk along the street, up there in the Overground, you see the first tendrils making their way up the ancient walls of the buildings. Neoprene-jacketed vines that grow in straight lines up sheer stone and masonry and inject themselves through holes in windowframes, homing in particularly on offices. Sometimes they are sheathed in metal tubes. Sometimes the owners have painted them over. But all of them share a common root system that flourishes in the unused channels and crevices of the Underground, converging on giant switching stations in deep bomb-proof vaults.