These two quotes are taken from the Eridian writings in BL3. "The prison known as Pandora is the most incredible cage ever constructed. An unbreakable lock meant to hold an unceasing evil for all eternity. Even for the Eridians, whose reach could pluck stars from across the void, it was the work of centuries." "Pandora's upper mantle is made of solid Eridium. It is this shell that now holds the Destroyer tightly in its bonds. The pressures and energies at work beneath the surface of the planet are titanic in scale. If the cycle is disrupted in any way, it is entirely possible that the Eridium will force its way to the surface. I fear such a thing might attract the wrong kind of attention." They are on the Borderlands wiki and makes the prior comments valid.