Looking very much like a special effect from a B movie, a funnel cloud of up to 1.5 million Mexican free-tailed bats swarms from under the Congress Avenue Bridge nightly from late March to early November. Turns out, Austin isn't just the live-music capital of the world; it's also home to the largest urban bat population in North America.
Austinites have embraced the winged mammals – figuratively speaking, of course – and gather to watch the bats' nightly exodus right around dusk as they leave for their evening meal. (Not to worry: they're looking for insects, and they mostly stay out of your hair.)
There's lots of standing room around parking lots and on the bridge itself, but if you want a more leisurely bat-watching experience, try Lone Star Riverboat or Capital Cruises for bat-watching tours.