Eternal Fabric is a block from Dimensional Doors mod. Found naturally in the lower levels in Limbo, and 4 blocks under the Fabric of Reality in Pocket Dimensions. It is unbreakable by any tool. When stood on (in Limbo, other dimensions unknown), it teleports you to the overworld at a seemingly random location with all of your items intact. This is one of the few ways to exit Limbo safely. Another way is to find a "pool" of Ancient Fabric.