Mexican free-tailed bats emerge from the Bracken Cave at dusk to foray for food on Tuesday, Sept. 19, 2017.  An estimated 20 million bats make Bracken Cave their home from March through November, making it the world's largest bat colony.  MARVIN PFEIFFER/ mpfeiffer@express-news.net Photo: Marvin Pfeiffer, Staff / San Antonio Express-News / Express-News 2017