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Approximately once in 3 real days, a rat nest appears (a small black location, which is needed to be found on WM).
Event lasts 5 hours, after which the nest disappears.
Every 20 minutes, rats sprawl along the radius from the nest. If a visible location is in this radius, then the rats enter there and begin to eat the population and all the incoming ones.
In the rat location, you need to kill a huge rat (Big Bos). In the location boxes, you can find random empty implant and shitty stuff like aramid, carbon, etc.