It all started in 1980, when the team of engineers designing the Congress Avenue bridge unknowinlgly created the crevices that would soon become the home to the largest urban bat colony in North America.

More than 1.5 million bats emerge each night at dusk, as they have for more than two decades, from the bridge's underbelly on food runs that blacken the otherwise red sky. Look at the chart below for the best times to watch the bats, or get directions and a map to the Congress bridge.