Underground

Beyond this last gate of the undergound lies an ocean of magma covering the surface world. It's flooding from a single borehole which used to provide the entire ecumenopolis with all of its energy. Then it was found that so much energy was being drained from the planet that it began to fall in on itself, causing earthquakes. A date was announced when the geothermal plant was to be shut down. Never having expected this to happen, its management had covered up the fact that it had failed to build a lock, which it only began to do after the announcement was made. On the date of the shutdown, the hastily built lock broke down, unleashing the now unused energy production of an entire planet as an eruption so great that even today, it hasn't stopped. We will never see the sky again. Even now, we're forced by earthquakes to move ever deeper as the magma flows through the faults.