London has always rested on a honeycomb of passages and voids, and each iteration of the capital seems to need more space beneath the earth. Even as other cities reach skyward or outward, London burrows down into the dark. Eight tunnels now run beneath the city in a grid, north–south and east–west, called respectively the "Warp" and "Weft" routes. These are the main roads that make transport within the overpopulated hub remotely possible; without them the city would freeze. This one is Weft 3, a favoured commuter journey from Oxford into the heart of the City. It is nearly seventy miles from one end to the other, and when first constructed required its own dedicated police and emergency unit, though many of these functions have now been subsumed by the Witness. In the event of a power outage—which is therefore engineered to be many times less likely than a direct meteor strike—the atmosphere inside would become lethal in a matter of minutes. It is a self-contained world and the tunnel must be lit dynamically or else the rhythm of the pools of shadow and illumination can induce pseudo-epilepsy in weary drivers. It is almost never closed. Today will be a very bad day for the capital’s commuters.