For anyone who doesn't know, Polybius is an urban legend about a government experiment in the form of an arcade game set up in Portland.

Here's the wiki article.

Polybius is a fictitious 1981 arcade game from an urban legend. The legend describes the game as part of a government-run crowdsourced psychology experiment based in Portland, Oregon. Gameplay supposedly produced intense psychoactive and addictive effects in the player. These few publicly staged arcade machines were said to have been visited periodically by men in black for the purpose of data-mining the machines and analyzing these effects. Supposedly, all of these Polybius arcade machines then disappeared from the arcade market.

It's a fun tale though, and there are some good articles and video essays on it.