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For several decades, a creepy urban legend has circulated in the darker corners of online forums devoted to vintage video games. The tale goes that in 1981, a game with some unfortunate side effects appeared in a few suburban arcades in Portland, Oregon. The game was said to have been housed in an all-black cabinet, and while playing it was fun, gamers soon noticed they were feeling terrible after their sessions—suffering from extreme anxiety, seizures, night terrors, and an obsessive desire to continue playing. Some were even said to have attempted suicide.

To make matters even weirder, men in black supposedly visited the cabinet every few weeks to collect some kind of data—not money—from the back of the machine. And just a few months after it appeared, the game was gone. Its name: Polybius.

Some said the game was connected to MKUltra, a (real) CIA program experimenting with behavior modification techniques and LSD from the 1950s through the '70s, although no evidence of that was ever found. Recently, Great Big Story's series "8 Bit Legacy: The Curious History of Video Games" set out to investigate Polybius, and found some surprising truths behind the mystery. They also found some fans attempting to recreate the game—hopefully minus the ill effects. You can learn more below:

This article was originally published on www.mentalfloss.com as Unraveling the Legend of Polybius, the Most Dangerous Video Game of the 1980s.