The first thing that always caught your eye were the covers. They stood completely apart from the fantasy-heavy YA books of the time. Each cover was like a magnet, drawing you across a school gymnasium during the yearly Scholastic Book Fair or leaving you staring slack-jawed in awe at the display in a bookstore window. The first one stands out in my memory, in particular: a boy looks out from the cover, utterly plain and ordinary in every way—except that he was slowly changing into a lizard through the magic of the finest rudimentary photoshop that 1996 had to offer. It was a startling revelation of a cover, fueling young imaginations for years to come.

There was absolutely nothing like K.A. Applegate’s Animorphs series back in the late ’90s and there may never be another series like it again. So why has it been largely forgotten?

Children’s books, before the groundbreaking advent of the YA genre juggernaut, tended to lean heavily into fantasy when it wasn’t about young women dying tragically (looking at you, Lurlene McDaniel). Science fiction was a rare find on those shelves, at least in my experience. The closest thing you would get were the slightly supernatural slasher novels of Christopher Pike, or R.L. Stine’s Fear Street series. If you wanted aliens or space ships you were completely out of luck. It seems strange, since the ’90s were huge for science fiction on the screen, but it took a surprisingly long time for the genre to hit the bookshelves in the young adult section with the same blockbuster force. K.A. Applegate arguably made the biggest splash with Animorphs, which quickly became a bestselling series—instilling in at least some of its young readers a set of lifelong fears involving ants, aliens, and authority figures.

Animorphs was absolutely perfect. It featured a diverse cast of characters, including various strong female characters, and was able to balance weighty real-life topics with the thrilling threat of an alien apocalypse. The series handles war in an incredibly adult way, filtered through the lens of aliens and high school. The characters are iconic and still well remembered to this day by a generation of young readers: the tragic figure that is Tobias, the badass that is Rachel, the charming alien friend Ax. The alien threat was vivid, silent, and frightening. It was a They Live! for the middle school kids of the ’90s mixed with the primal wish fulfillment of being able to transform into animals. It balanced horror and humor on a fine knife’s edge, keeping young readers on their toes and awake all night. Ask any hardcore Animorphs fan and they’ll immediately be able to name at least one thing that delighted them about the series along with something that caused some minor psychological trauma (or perhaps just occasional nightmares) for years to come—these books had everything!

And yet, the series seems to have been lost to time. Despite being hugely popular, it never achieved the success or the staying power of, say, the Harry Potter books. Animorphs tried hard to break into other mediums, including a cringeworthy TV series that aired on Nickelodeon, but never stuck the landing. It burned bright and then faded away, racking up a troublingly high body count in its final installments. Scholastic tried to re-release the series in 2011 but was met with tepid interest. In a world of bleak YA novels that seem to offer readers an endless game of Choose Your Own Dystopia, the Animorphs books seem somewhat quaint in comparison. Despite this, for those of us who grew up with the series, just seeing those iconic covers again is like being punched in the sternum by nostalgia.

Animorphs made such an immediate and lasting impression on its target audience because the characters felt like actual teenagers, and the problems they encountered felt organic and realistic. Even when the series was dealing with death or abuse it did so with a down-to-earth vibe that never felt like an After School Special. The books tackled difficult topics like death, depression, drug abuse, parental neglect, and bullying with an air of care and compassion. The aliens, called the Yeerks, were legitimately scary and were depicted in a way that made them feel like a real threat—I can’t tell you the number of nightmares I had because of them. They were small, slug-like creatures that would take over your body by entering your ear canal and nesting in your brain. Anyone could be a Yeerk: your principal, the police, your parents. If that premise doesn’t keep you up at night, you are made of stronger stuff than I am. The aliens opposing the Yeerks—and who gave our team of intrepid teen heroes the ability to shapeshift—were strange, deer-centaur-esque aliens called Andalites. They were wise and ethereal, absolutely bizarre and otherworldly. Between them, the Yeerks and the Andalites form the compelling sci-fi core of the the world K.A. Applegate built.

The series was completed in 2001, and sprawls out across almost sixty books. K.A. Applegate (actually husband-and-wife writing team Katherine Applegate and Michael Grant) wrote roughly half of them. The rest were crafted through the time-honored tradition of ghostwriting, with other writers working from a detailed outline provided by Grant and Applegate, under their supervision. So many book series for young adults were built this way and it’s been a successful strategy. Carolyn Keene, the author credited for the Nancy Drew mysteries, straight-up never existed, for example. Ann M. Martin, the author behind the insanely popular Baby-Sitters Club series, claims to have written less than half of the books. This was a viable way to keep up the grueling pace of children’s publishing, particularly in the ’90s. Back in those days, books for children and teens came out with headspinning frequency. Animorph books came out quarterly, each clocking anywhere between 150 to 200 pages. The breakneck pace was a boon to hungry fans with allowance money burning a hole in their pocket but required a stable of ghostwriters in order to meet the demand. Adult genre fans are used to waiting years for the next volume in their beloved series. Imagine George R. R. Martin releasing a Song of Ice and Fire book every three months! (No, wait, don’t imagine that. Oh no, stop crying, I’m sorry. The Winds of Winter will come out some day, I promise!)

You can’t really call Animorphs timeless, honestly. They take place in a kind of ’90s pop culture milieu that simply doesn’t exist anymore. The Yeerks invasion would have never worked in our world of ubiquitous social media and constant surveillance. That’s part of the charm now. Can you remember a world without Twitter notifications and incessant email pings, where your biggest concern was if you should start the latest Animorphs book before or after the new Legends of the Hidden Temple episode? What is timeless, however, are the characters. They felt like your best friends and plunged into breathtaking adventures on their quest to save the world. Jake, Marco, and Tobias were each amazing in their own way, but the real stars were Cassie and Rachel.

When it came to female heroes that were easy to identify with in most YA books in the ’90s, I always found the pickings to be rather slim. You could try to see yourself in the conniving fashion plates that populated the Sweet Valley High books, or maybe imagine yourself dying of some terrible disease while your true love watched, or being murdered by a serial killer in a Fear Street book. TV was a little better, with Sarah Michelle Gellar’s slayer heroine in Buffy and the hard-fighting women of Xena: Warrior Princess reigning supreme, but the closest thing you got in young adult fiction were the brave women of Tamora Pierce’s fantasy novels (which I wrote about here).

If you didn’t like dragons and knights, though, you were out of luck…that is, until Animorphs’ Rachel shifted into a bear and kicked the door down. She was the epitome of a badass, brave in the face of danger and skilled in battle. Cassie, on the other hand, was an environmentalist and a wary participant in the alien war. She had the strongest shifting ability of the team—as an “estreen,” her natural morphing abilities allow her to approach the level of an artist when turning herself into different creatures. Cassie and Rachel were two sides of the same coin: strong and fearless women tossed into a war, trying to survive as best they could with their values and sanity intact.

The series was fraught with heartbreak and loss. Tobias is the first casualty, after staying in his hawk form over the two hour morphing time limit. He spends the entire series trapped in that form, having forfeited his human shape. I cried so many teenaged tears for him. His mistake, the plot of the first book, sets the tone of the series. There would be humor and adventure throughout the books, but there would also be terrible and unfair tragedy. Tobias was still alive, at least. It’s Rachel who makes the ultimate sacrifice, dying at the end of the series in order to prevent the alien infestation from spreading. She’s honored as the hero she is when the series ends with the human race learning about the alien threat—and the teenaged heroes who had been courageously holding it at bay for so many years.

I faithfully read every book in the Animorphs series, constantly bugging my parents to get me the latest volume when it came out. I dutifully circled the books in red on the Scholastic Book Fair order forms. I asked for them every Christmas and managed to weasel a few extra volumes during long family road trips. My teachers confiscated them when I tried to sneak in a few chapters during science class, and I once turned in an ill-fated Animorphs-inspired diorama—one that involved pipe cleaners and very sad construction paper-monstrosities that I had the gall to call “animals”—to my very bewildered English teacher. These books shaped my adolescence and instilled in me a real love of science fiction. For the longest time I had thought that I only liked fantasy stories, and I barely read anything modern. Animorphs lit a fire in me for science fiction that burns to this day. They were a stepping stone that led to grabbing the likes of Michael Crichton and Ray Bradbury from the adult shelves at the library.

Animorphs will always have a cherished place in my heart—a place that feels eternally like summer vacation and smells like a school gym full of fresh new books. Those of us who fell under the spell of these books as kids know that we need to keep them alive—after all, the war might not be over… Who knows who might be a Yeerk or not? I still sometimes wonder, decades later!

Animorphs were a whirlwind of a series, one that left a mark stamped on every reader drawn into their world. It’s a shame the books have all but vanished—they would make an incredible Netflix series, and CGI is so much cheaper (and better) then it used to be. I’m convinced that today’s technology would allow for some really cool and inventive approaches to adapting these stories. With the push of ’90s nostalgia stronger than ever right now, I have my fingers crossed that someone will revive the series properly—I, for one, would much rather see Animorphs come back into style then scrunchies. K.A. Applegate did an outstanding job building a world that held real meaning for a generation of readers, and sixty books is nothing to sneeze at. The characters Applegate created still live on in the minds of the series’ fans—and those bright, jarring, iconic covers are still some of the coolest, weirdest things we’ve ever seen.

Meghan Ball is an avid reader, writer, and lifelong fan of science fiction and fantasy. When she isn’t losing to a video game or playing the guitar badly, she’s writing short fiction and spending way too much time on Twitter. You can find her there @EldritchGirl. She currently lives in a weird part of New Jersey.