What) The Ann W. Richards Congress Avenue Bridge crosses over Lady Bird Lake in Austin. The bridge is a concrete arch bridge, with three southbound and three northbound vehicle lanes and sidewalks on both sides.
The bridge is also home to the world's largest urban bat colony which is composed of Mexican free-tailed bats. The bats reside beneath the road deck in gaps between the concrete component structures. They are migratory, spending their summers in Austin and the winters in Mexico. According to Bat Conservation International, between 750,000 and 1.5 million bats reside underneath the bridge each summer. The nightly emergence of the bats from underneath the bridge at dusk, and their flight across Lady Bird Lake primarily to the east, to feed themselves, attracts as many as 100,000 tourists annually. Wear a hat.
Another convenient-for-tourists (but smaller) bat colony is reported 15 miles north of Austin, underneath the I-35 underpass at McNeil Road in Round Rock. Wear a hat there too. (1)
If you are interested then you can click here to see a couple of the information plaques that I photographed.
Where) 305 South Congress Avenue free parking to the public after 6 pm. March through November.
Why) Even
though our arrival date in early March meant that we would
likely miss the bats it did seem likely that we would enjoy
looking at the bridge and walking along the lake. 
... the water running underneath the bridge.
(3)
