Tartarus Prime's odd composition is attributed to the fact that it didn't form naturally but was put together with billions upon billions of asteroids moved by the . This planetoid was built as a prison to encase "It" and even succeeded for several million years but when excessive mining created weak points in the giant shell, It was released. In It's escape from Tartarus Prime, the planetoid was nearly obliterated and every mining platform along with their occupants were destroyed in It's fury.