NYSSA: But there would be records. A people eight hundred years ago capable of molecular engineering?

DOCTOR: Not necessarily. I suspect that when they built the Great Crystal they overlooked one vital factor. The nature of the mental energy would determine the nature of the matter created. The Great Crystal absorbed what was in their minds. The restlessness, the hatred, the greed. Absorbed it, amplified it, reflected it.

NYSSA: And created the Mara.

DOCTOR: Indeed. And in the reign of evil which followed they must have forgotten the most important thing of all, that the Mara was something they themselves had blindly brought into being.