Morlocks

The X-Men's first encounter with the Morlocks occurred when Callisto had Angel kidnapped, intending him to be her husband. The X-Men entered the tunnels, and upon finding the Morlocks were captured. Worse yet, Kitty Pryde was infected by Plague and hovered near death. Following the rules of the Morlocks, Storm, although herself suffering from Plague's touch, challenged Callisto to single combat. The winner would get the lives of all the captives, and the right to lead the Morlocks. The loser would die. Storm fought well, and stabbed Cal in the heart. Luckily, the Morlock Healer was able to save her. Storm became the leader of the Morlocks, and offered to take them up into the world, but they declined.

Months later, the tunnels were broken into by Gambit, and the Marauders commenced a slaughter of those mutants living there. The X-Men came to help, but were gravely injured, and only managed to save a few Morlocks, most of whom were then transferred to Muir Island. The remaining Morlocks fell under Masque's control, and his constant manipulation of their bodies drove many insane.

The Morlocks who went to Muir were less lucky: the island was soon attacked by the Reavers, and many more of them died.

After Masque's death, the remaining New York Morlocks went completely insane, and managed to escape the tunnels, wreaking havoc in Times Square until the X-Men were able to subdue them. Unfortunately, upon their return to the tunnels, Mikhail Rasputin, himself crazy with grief over his lost family, flooded and collapsed the tunnels, apparently killing himself and the Morlocks.

In reality, however, Mikhail had used his power to open a portal to a dimension called "The Hill," where time flowed faster, and where the next generation of Morlocks quickly grew up, each member vying for survival in that harsh climate. They eventually returned to our world as Gene Nation.

Dark Beast also has some connection with the Morlocks. Most likely he was using them for his genetic experiments.