No one seems to know the origin of Georgia Pacific’s signature approach, the Bat, a long-form improvisational technique performed entirely in the dark and bookended by soundscapes the ensemble generates. “The basic exercise is not a mystery,” says troupe member Lisa Lewis. “In some ways, it’s like the best of old radio–it’s really about heightening the theater of the imagination.” Perhaps it’s best that the Bat retain an aura of inscrutability: it’s like a vision quest that shouldn’t work but somehow does, opening up the other senses while shutting down sight. Beginning as an ImprovOlympic Harold team in 1995, Georgia Pacific performed there for seven years–a few of them with the houselights on–then some of its members moved to New York and LA. Lewis insists the troupe never really broke up, and the members have performed a few times together in the last three years. Still, this reunion at the Chicago Improv Festival is a rare treat: in the olden days the team riffed on the Annoyance style with dark, often sardonic scene work and an emphasis on quickness and verbal gymnastics. The Reckoning–part of ImprovOlympic’s next generation of Harold performers–is the opening act. Tue 4/26, 8 PM. Theatre Building Chicago. $10.