According to a new report published in the journal Science, Bats slam into vertical structures such as steel and glass buildings because they appear invisible to bats' echolocation system. Nature reports: Bats rely on echolocation to navigate in the dark. They locate and identify objects by sending out shrill calls and listening to the echoes that bounce back. Greif and his colleagues tested the echolocation of 21 wild-caught greater mouse-eared bats (Myotis myotis) in the lab. The researchers placed a featureless metal plate on a side wall at the end of a flight tunnel. The bats interpreted the smooth surface -- but not the adjacent, felt-covered walls -- as a clear flight path. Over an an average of around 20 trials for each bat, 19 of them crashed into the panel at least once. The researchers also put up smooth, vertical plates near wild bat colonies, and saw similar results. The animals became confused owing to a property of smooth surfaces called "acoustic mirroring." Whereas rough objects bounce some echoes back towards the bat, says Greif, a smooth surface reflects all echolocation calls away from the source. This makes a smooth wall appear as empty space to the bats, until they are directly in front of it. Only once a bat is facing the surface are their perpendicular echoes reflected back, which alerts the bat to its mistake. This explains why some bats attempted to swerve out of harm's way at the last second -- but often too late.