THE DOCTOR: "Why is there a patch of earth on the middle of your floor?"
NASREEN: "We don't know, it just appeared overnight."
THE DOCTOR: "Good, right, you all need to get out of here very fast."
Over in the tiny, remote hamlet of Cwmtaff, human science is again being bold, pushing boundaries and asking for trouble. The Discovery Drilling Project- or as the technically-minded Doctor calls it, "a Big Mining Thing"- has burrowed 21 kilometres into the Earth, deeper than ever before. However as Discovery boss Nasreen Choudhry and her project manager/unrequited love Tony Mack celebrate, an ancient race launch pre-emptive strikes, from way further beneath their feet than 21 kilometres. People, alive and dead, start to vanish. The Doctor, Amy and Rory face the ultimate uprising...