Once in a while you'll get a message that says "a space/time distortion appeared!" and an area on the map will have a "force field" around it. If you walk through, it'll be full of strong Pokemon trying to kick your ass and relentlessly chasing you around, but it's also loaded with items you can pick up. Pokemon who normally aren't supposed to exist, like Magnemite and Porygon, spawn during them.
