Board Thread:Questions and Answers/@comment-98.237.146.5-20130323225857/@comment-3552584-20130323232023

It's a rift, from the Dimension Doors mod. you can remove it with the Rift Remover tool, or place a Dimension Door on it to contain/open it. for the one in the picture, you'll need to place a solid block before you can place the Dimension Door. I personally haven't fully explored all that you can do with that mod, so I may be missing some information.