Board Thread:Questions and Answers/@comment-92.238.197.187-20130728202324/@comment-13413401-20130728204253

The black bubbly thing is called a rift. If you were to place another dimensional door on the block under it (so the door takes up the rift), you would go to the pocket dimension that existed when the door was there before.

To remove it permanetly, you will need to use a rift remover. However, you could remove it temporarily with TNT.

WARNING: You should remove it as soon as possible if there are other rifts like this nearby! If you let it stay, the rifts will spread and create many more, which you probably dont want in your base... If there are no rifts nearby, you could wait to remove it.