By day, the Congress Avenue Bridge in the city of Austin could hardly look more normal: a grey, dreary city-centre road bridge. By night, it plays host to one of the most amazing shows nature has to offer. The underside of the bridge is home to more than a million bats, and every evening in summer they all come swarming out at once, rising up into the city sky like a tornado before spreading out in all directions like plumes of smoke. Standing on the bridge, you might even feel the wind from their wings as they pass by.