Some Pokemon Go players think they've unlocked the secrets to getting an EX Raid to appear at certain gyms.
Last fall, Pokemon Go introduced the EX Raid, a new type of raid that players can only participate in IF they have an invite. The raid invite gives players a time and location where they can exchange their invite for a chance to catch and battle the Legendary Pokemon Mewtwo.
On average, invites to EX raids go out about once a week, but only to players who previously completed a raid at the location where the EX Raid is to take place. There's no way of knowing where an EX Raid will take place, so many players have complained loudly about the seeming randomness of how, when, and where the raids appear. However, some players believe that they have unlocked the secret to guaranteeing that an EX Raid appears at a specific spot.
There are two somewhat conjoined theories that these players are drawing from. The first uses OpenStreetMap (the mapping system Pokemon Go uses) and "s2 cells" (a geometric tool used by Google and other mapping systems to divide maps into easy to index reference points) to determine what gyms are eligible for an EX Raid to appear at. According to the theory, EX Raids will appear only at sponsored gyms (Sprint or Starbuck locations in the United States) or locations that are coded as parks on OpenStreetMap.
Additionally, a player on TheSilphRoad subreddit discovered that Niantic "distributes" EX raids at a rate of one EX Raid per 12th level s2 cell and that a location will never host an EX raid on two consecutive raid windows when possible.
By using these two criteria, the theory posits that players can figure out which raids to focus on by cross-referencing local gyms with OpenStreetMaps and an "s2 cell." If a 12th level s2 cell only has two eligible gyms, a player simply needs to focus their raiding efforts on whichever gym didn't last have an EX raid to maximize their chances of getting an EX Raid Pass.
The second theory is that players can collectively "trigger" an EX Raid at a specific location by bombarding it with activity. This theory states that by organizing well-attended raids at a specific eligible location, they can somehow rig Niantic's algorithm to increase the odds that an EX Raid will appear at that gym.
While the first theory is pretty well-supported by evidence, the theory that players can somehow rig Pokemon Go to get an EX Raid at a certain spot is a lot more shaky. After all, most s2 cells will only have a handful of eligible gyms, so it's possible that players might have just gotten lucky with the odds.
We're still trying to understand how EX Raids work, but it seems that a Pokemon Go player can "crack the code" with a bit of digging. You can check out how your neighborhood is divided by s2 cells here and see how your neighborhood appears on OpenStreetMap here.