Planet X
From Memory Alpha, the free Star Trek reference
Planet X was a fictional planet featured in chapter 18 of Tom Paris' holodeck program The Adventures of Captain Proton, home to Doctor Chaotica's Fortress of Doom. As a nod to the practice of re-using sets in the 1930s serials Captain Proton was emulating, Planet X looked almost identical to the mines of Mercury, though it lacked an erupting volcano. (VOY: "Bride of Chaotica!")
