Was leveling my warlock thru the story, and using the glide managed to sneak between the death rotators in the almighty, and get under the platform! Getting in is easy, it's getting out alive that's a problem! You could prob even get in with any other class.
How to do it!
You'll need to be on the almighty, and get to the part with the rotary crushers
https://imgur.com/BWLngl9
This is the part specifically. We want to jump dowwn, run on the death crushers, and hop over a small ledge, and boom! We're under the floor.
https://imgur.com/EL3bOlK
This is the ledge we'll be jumping over, you can stand on it. Once on the other side, you can run back and forth as you please! Don't sink too low, or you'll burn to death!
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I had the game spawn me outside of the map when joining my wife on IO one time.