Large lava pit arena

In a 2010 interview with Mike Sneath, it was revealed that Thardus was originally slated to appear in a large lava pit arena, somewhere in the Magmoor Caverns in Metroid Prime, before it was changed to the Quarantine Cave in Phendrana Drifts, because of time constraints.

Interview
...Thardus was concepted by that first concept artist I mentioned earlier. There was a movie that inspired this character and um I don’t remember the movie. Sorry man, seems like a long time ago. I am not 100% sure but if I remember correctly Thardus was suppose to be in a large lava pit arena but I believe we had to change it due to time constraints. There’s not a whole lot to say about this boss since it’s a bunch of rocks so I looked at rocks for additional inspiration. I would say of all the bosses for Metroid this one was the easiest for me to create."