A Power Tower was a power station in Mega-City One which drilled down to the magma beneath the Earth's crust, extracting the energy to fuel the city. Geothermal power was carried from beneath the crust to the city by plasteel tubes which ran up through the Undercity. It was a source of geothermal energy for more than half of the city. Molten lava was drawn lave from the Earth's core which was contained by a computerised magnetic field system.