"Catacombs"
Someone has taken Warren Worthington III and the X-Men are called in to investigate. The trail leads underground where Storm, Colossus, Nightcrawler and Kitty encounter an underground civilization, but Kitty is separated in the ensuing battle and ends up being rescued by Caliban. Meanwhile, Storm, Colossus and Nightcrawler find Warren, who has been taken to be the King of the Morlocks and their Queen, Callisto.
First appearance of the Morlocks.