Prepare to enter The Matrix with these 15 little-known facts about the Wachowskis’ mind-bending 1999 film, which was released 20 years ago today.

1. The Matrix began as a comic book.

Filmmakers Lana and Lilly Wachowski originally conceived the storyline for The Matrix as a comic book. They had both previously written comic books for Marvel.

2. The unnamed city you see in The Matrix is Sydney, Australia.

The production shot the entire film—both interior sets and exteriors—in Australia for tax purposes, significantly lowering the film’s budget. However, all the street names are taken from locations in Chicago, where the Wachowskis grew up.

3. The studio didn’t want the Wachowskis to direct.

Warner Brothers originally thought the Wachowskis, who had no directorial experience, were unqualified to direct The Matrix. To prove their mettle, the Wachowskis wrote and directed the crime thriller Bound, which became a modest hit. And convinced the studio that they knew what they were doing.

4. Martial arts choreographer Yuen Woo-ping created the film’s elaborate stunts and wirework.

The Wachowskis specifically enlisted Yuen Woo-ping’s talents because they loved his work on the 1994 Hong Kong martial arts film Fist of Legend.

5. Both Will Smith and Nicolas Cage formally turned down the role of Neo.

Warner Home Video

Will Smith said no so that he could make Wild Wild West. When asked by Newsweek if there were any roles he regretted turning down, Nicolas Cage replied that, "I don't really have any regrets. I think regret is a waste of time." Yet he allowed that there "were movies that I probably would have benefited from if circumstances in my life allowed me to make them," and cited both The Matrix and The Lord of the Rings. Tom Cruise, Johnny Depp, and Leonardo DiCaprio were all also reportedly considered before the filmmakers settled on Keanu Reeves for the part.

6. Russell Crowe, Sean Connery, and Samuel L. Jackson could’ve been Morpheus.

For Sean Connery, it was apparently a matter of not understanding the script (which was also the reason he turned down The Lord of the Rings). When they declined, Laurence Fishburne took the role.

7. The actors were asked to brush up on their knowledge of philosophy before production began.

The Wachowskis had all the lead actors read Simulacra and Simulation by Jean Baudrillard, Out of Control by Kevin Kelly, and Introducing Evolutionary Psychology by Dylan Evans and Oscar Zarate in order to better understand the world of the movie. In the film, Neo actually hides his illegal computer files in a copy of Baudrillard’s book.

8. The movie is color-coded.

Every scene that takes place within the computer world of the Matrix was given a green tint, while all the scenes that take place within the real world have a blue tint. In fact, the only time the color green appears in the real world-set scenes is in the Matrix code on the ship’s computer screen.

9. Hugo Weaving didn’t have to look far for inspiration for his character.

Warner Home Video

He modeled Agent Smith’s voice after the Wachowskis themselves.

10. The lead actors trained every day for four months in order to pull off the fight scenes.

Just like the Hong Kong martial arts movies that influenced them, the Wachowskis wanted the actors—not stunt people—to be the ones fighting onscreen.

11. Keanu Reeves had cervical spine surgery prior to the training period that forced him to wear a neck brace throughout.

This rendered it impossible for Reeves to kick effectively, so Yuen Woo-ping had to adjust his choreography accordingly. In the finished film, Neo hardly kicks at all.

12. Hugo Weaving had to undergo hip surgery after being injured during fight training.

This completely shifted the shooting schedule, and Weaving’s fight scenes were completed at the end of the production in order to allow time for him to heal. All-in-all, it was a tough shoot for all the main actors. "Hugo had hip surgery. Carrie-Anne [hurt] her hip and ankle. Laurence got hit in the head. He got his eyelid sliced open ... I couldn't walk a couple of times," Reeves told The Guardian. "I mean, I'm exaggerating a bit, but there were a couple of sequences where I had to carry all these guns. It was about 50 pounds of weapons. And waiting for the special effects sequences, the bullet hits, etc.—there was one time where we stood for, like, three hours."

13. Neo and Trinity’s three-minute lobby shootout took 10 days to shoot.

No CGI was used—all of the explosions and gunfire were practical effects.

14. The sunglasses for each character were custom-designed by Blinde Design.

They weren’t available to purchase until the film’s sequels (The Matrix Reloaded and The Matrix Revolutions) were released.

15. The iconic “Bullet Time” effect was conceived and created specially for The Matrix.

The famous swirling shot of Neo’s gravity-defying backbend was made using a rig that contained 120 individual digital still cameras and two film cameras. The still images were carefully stitched together to create the shot frame by frame. The first test shot of the Bullet Time effect gave a nearly 360-degree view of an exploding trash can.

An earlier version of this article ran in 2015.