That’s what the black doorway with the purple swirls is in the screenshot above. You make one by building a frame out of obsidian – a hard and rare block type already in the game – then setting fire to the hole you’ve created; an awesome way to get anywhere. When you step inside, an identical portal will be created at the equivalent spot in the hell dimension, unless there’s already one somewhere near. You pop out of that, and hope there aren’t any Ghasts around.