If you're an X-Fan of the '90s, then last week's "X-Men '92" #5 jolted you like a can of Surge. That series has mastered the art of '90s nostalgia, but that issue's last page reveal took the retro mutant book to a gnarly new level. The final page of the issue included the return of a team that hasn't been seen in almost twenty years: the X-Men of 2099.

Yes, somehow the mutants of the '90s series "X-Men 2099" have stayed off the radar since 1998 -- aside from a brief, almost entirely unrecognizable cameo appearance in 2014's "Uncanny X-Men Annual" #1. But the team, as illustrated by Cory Hamscher, on the last page of Chad Bowers and Chris Sims' "X-Men '92" #5 was unmistakably the X-Men of 2099 -- leather jackets, head socks and all!

But who were the X-Men of 2099? Considering that they haven't headlined a series since the end of their ongoing in 1996, it's entirely likely that there are plenty of full-grown comic readers out there that missed out completely on Skullfire, Bloodhawk, Meanstreak, Metalhead and all the other compound code-named heroes. If the last page of "X-Men '92" #5 left you scratching your head, here's a refresher on the X-Men of the future from the '90s.

What Is 2099?

First thing's first: the number 2099 refers to the year. "X-Men 2099" was part of a line of comics published by Marvel set in the future year. While the line formally launched in late 1992, its origins actually stretch back 1990 and involve two major Marvel players: Stan Lee and John Byrne. As Zedric Dimalanta broke down in a detailed 2099 retrospective, what became the 2099 line started out as a graphic novel called "The Marvel World of Tomorrow" to be co-written by the two with art by Byrne. Byrne eventually left the project and took all of his contributions with him, leaving Marvel with enough bits and pieces to mold into what would become the 2099 line.

"Spider-Man 2099" from Peter David and Rick Leonardi kicked things off in November 1992, and another new 2099 title launched each following month. "Ravage 2099" came a month later, featuring the hero Stan Lee created for his partnership with Byrne, this time drawn by Paul Ryan. Then John Francis Moore and Pat Broderick launched "Doom 2099," and "Punisher 2099" from Pat Mills, Tony Skinner and Tom Morgan rounded out the initial 2099 wave. All four books were set in the same cyberpunk-fueled dystopia, packed with early '90s political commentary on the relationship between America and corporations. And despite being set 100 years in the future, nearly every character wore the spikes, chains and shoulder pads so common in the comics of 1992.

Considering how enormously popular the X-Men were in 1992 ("X-Men" #1 had sold 8.1 million copies a year before and an animated series was set to debut in October on Fox), it may be surprising to learn that they were not part of the initial wave of 2099 titles. Aside from Stan Lee's creation Ravage, the other three 2099 books all starred heroes picking up the torch from a known Marvel character (or, in the case of Doom, actually was the Victor Von Doom fans knew). Things would be different for the X-Men.

Enter: X-Men 2099

"X-Men 2099" #1 arrived in stores in October 1993, eight months after the launch of the 2099 line. It also arrived alone; the next 2099 book -- "Ghost Rider 2099" -- wouldn't debut until six months later. Even though fans had yet to read a single page of "X-Men 2099," "X-Men 2099" #2 still had a letters page packed with readers writing in demanding that Marvel make an X-Men 2099 comic.

"I'm very disappointed with you, young men, go to your writing rooms!" reads one such fan letter from "X-Men 2099" #2, mailed in anonymously. "You forgot everyone's favorite mutants, the X-Men, when you made 2099 comics."

The X-Men's arrival on the 2099 scene also stood out from much of what had come before; unlike Spider-Man, Doom and Punisher, the X-Men were not clearcut, futuristic analogues of previously known Marvel heroes. Writer John Francis Moore, artist Ron Lim and line editor Joey Cavalieri instead created a totally new generation of young mutants, all of whom were unrelated to established X-Men. Here's the team that was introduced in 1993's "X-Men 2099" #1:

The Rise & Fall of "X-Men 2099"

The X-Men could do no wrong in the early-to-mid '90s. Powered by record-breaking sales success and a blockbuster Saturday morning cartoon, the mutants found the kind of love and acceptance in the real world that they fought for in their stories. They were on everything form pizza boxes to pogs, and even a comic book starring zero known characters (like "X-Men 2099") was a guaranteed hit. In "X-Men 2099" #18, the series published its first statement of ownership that detailed its circulation numbers. The average number of copies of a single sold up to that point was 368,867. To put that in perspective, "X-Men 2099" #1 sold around 130K more copies than "DC Universe Rebirth."

Return To 2099

The X-Men of 2099 pretty much laid low for the next few decades. Halloween Jack made a one-off appearance in "X-Force" in 1999. A 2009 limited series titled "Timestorm 2009-2099" featured a dramatically redesigned take on the 2099 universe and parallel versions of characters like Cerebra, Meanstreak, Krystalin and Bloodhawk. Artist Andrea Sorrentino presented a drastically modernized version of the team -- seen above -- for a very brief scene in 2014's "Uncanny X-Men Annual" #1.

For a team whose series lasted 35 issues and got the action figure treatment, it's a little surprising that the only time these characters have popped up since 1998 were in nearly unrecognizable cameo roles. That's why their sudden reappearance at the end of "X-Men '92" #5 struck a major nostalgia chord; these characters have barely been seen in almost 20 years, and they definitely haven't been seen in their classic looks since 1998.

"X-Men '92" co-writer Chad Bowers discussed the decision to dust off the 2099 team. "One of the things we wanted to do with the ['X-Men '92'] ongoing was to have the X-Men, who were sort of sequestered off in their own little corner of he Marvel U in the '90s, dip their toes into the other hyped-up lines of the era, and show what those crossovers might've looked like," said Bowers. "So Midnight Sons spills into the first arc. 2099 becomes a nice layover for the time-tossed Scott and Jean. Marvel UK's next!"

Bowers' co-writer Chris Sims added: "Like Chad said, part of what we wanted to do with 'X-Men '92' was to explore everything that made the '90s what they were, including the weirder corners like the Midnight Sons, Marvel UK, and, of course, 2099, the imprint that gave us the immortal line 'I don't need a jetpack -- all I need is HATE!' Besides, those books are fascinating to me. They're built around concepts that aren't just weird, but they're that special kind of bizarre that really makes them seem like they're from the future -- although maybe not a hundred years. I mean, think about it -- stuff like people worshipping Thor as a full-blown religion, or a possibly time-traveling Dr. Doom taking over the country and ruling over America? Those are ideas that would be right at home in 2016, and that makes those characters really fun to play with."

After their last-page return in "X-Men '92" #5, it's not yet known when the future-set narrative of that series will pick back up again. As solicited, issues #6-8 appear to cover the exploits of the main X-Team. But however much time passes before we pick back up with Cyclops, Jean Grey and the X-Men of 2099, odds are it'll be much sooner than 18 years.