For some, the X-Factor comes with changes too drastic to be hidden from the rest of the world. Whether it is obvious physical changes that would cause them to be shunned in the normal world, or inconvenient powers that keep them from interacting with most of society, these outcasts are left without many places they can turn.
Many of them have banded together to form the Morlocks, a loosely-organized group who lives on the fringes of society, slumming in abandoned buildings and subway tunnels, sometimes stealing food and money to survive. Heard of among mutant subcultures mostly through rumours, no one knows for certain where exactly they can be found, but if someone who needs to join their ranks looks hard enough, they'll be likely to find them.
A draft of the various subterranean NPCs can be found here.
A note on visiting the Morlocks: The Morlocks dislike outsiders, by and large. Including other mutants, most times! Their tunnels are a labyrinth riddled with traps and patrolled by guards both PC and NPC, and they don't take kindly to unannounced visitors. If you are not a Morlock, please do not roleplay in the Morlock tunnels as if your character is freely wandering/exploring their home. If your character is searching for the Morlocks -- whether out of morbid curiosity, bravado, fascination, or an desire to join them, feel free to ping a Morlock PC or a staff/guide to NPC one of the Morlocks they would undoubtedly encounter long before finding their actual home.
A note on joining the Morlocks: The Morlocks are a pretty insular and clannish group. Unlike some other factions, they aren't joined together so much by cause or ideology but moreso by both necessity (being largely unable to assimilate in any way into the rest of the world) and a desire to protect their own -- not mutantkind, but Morlocks. They are highly suspicious of strangers, even other mutants, and guard their domain fiercely and violently.
With that said, mutants who do seem like they could use a home in the Morlocks' world can often find a welcoming home among them. The best way to do that is to score an invite into the Tunnels from another Morlock, as they're usually the ones who know how to navigate the mazelike tunnels without setting off any of their myriad traps.
Please make sure that if your character is joining the Morlocks you also let game staff know OOCly so that they can help facilitate faction change process!