Among perhaps hundreds of Easter eggs in 'Ready Player One', this one was totally real.

There’s a hugely important Easter egg hidden in the OASIS of Ready Player One, but the most important Easter egg in a movie saturated with them also exists here in the real world too.

The OASIS, James Halliday, and his crazy contest might all be fiction created by Ernest Cline. But the story of Warren Robinett, Adventure, and the world’s first Easter egg are all totally real.

Spoilers follow for the epic conclusion to Ready Player One, in both book and movie form.

In Ready Player One, OASIS creator James Halliday’s last will and testament is presented in a video, called “Anorak’s Invitation,” that launches the contest to win his wealth and control over the OASIS. In the video — from the book at least — Halliday explains how game designer Warren Robinett didn’t get any credit for creating the Atari 2600 video game called Adventure. So Robinett programmed his name into a secret room in one the game’s labyrinths. Players could only access it if they found a specific key and used it in a special place.

It seems pretty wild, except that part is 100 percent real.

James Halliday (Mark Rylance) as he appears in the 'Ready Player One' movie.
James Halliday (Mark Rylance) as he appears in the 'Ready Player One' movie.

Atari shipped Adventure in 1979 (1980 by some accounts) without realizing Robinett programmed this into the actual game. Nobody found out until months later when kids all over the world discovered the world’s first-ever Easter egg. “I was one of those kids,” Halliday says during Anorak’s Invitation, “and finding Robinett’s Easter egg for the first time was one of the coolest videogaming experiences of my life.”

Adventure also happens to be the world’s first-ever console adventure game. In it, a knight explores maze-like dungeons in search of a golden chalice, fighting three roaming dragons and a pesky bat along the way. You grab keys and other items to unlock new areas. But everything mostly just looks like tiny dots or pixelated objects.

Even today, you can play different emulator versions of Adventure and discover the Easter egg for yourself. You just have to carry a bridge into a black castle, which reveals a secret room where you can pick up a seemingly invisible key. Carrying that to the main hallway of the game opens up a passage to the secret room that holds the Easter egg. Easy enough, right?

Here’s that room and the Easter egg in all its glory:

This is what you get from the super-secret Easter egg in 'Adventure'.

Ultimately, it’s all too fitting that everything in Halliday’s Contest hinges on Adventure. The contest itself sends millions of gamers on a quest as they use their knowledge of Easter eggs from across tons of different video games, shows, and movies just to find a single Easter egg in the digital world of the OASIS. Given the nature of the Easter egg itself, it also makes sense why Halliday was so obsessed with keys as part of the contest.

The circumstances in which Wade plays through Adventure at the end of the Ready Player One novel are vastly different than in the movie, but in both versions the game is hugely important and one of the very last things he has to do as Parzival to claim the Egg.

Tons of what you see in Ready Player One is total fiction, but this specific part that inspired James Halliday and even author Ernest Cline, is totally true.

Ready Player One hits theaters March 29.