Spandau Citadel: Berlin, Germany
Completed in 1549, Berlin's Spandau Citadel is one of the best-preserved Renaissance fortresses in Europe. But under the citadel's vaulted roof lives a spooky secret: the place is actually home to one of Europe's largest bat colonies, who have spent their winters cozying up in the citadel since its completion. Visitors can check out the 10,000 bats from a separate viewing room, or take a guided tour, offered from summer to early fall, when the bats begin returning for the winter.
Interested in learning about other locations to see bats? Bat Conservation International has created a nifty map of other bat viewing spots around the world.