At dusk, 20 million Mexican free-tail bats would pour from the darkness inside Bracken Cave on the outskirts of San Antonio, Tex., in a skillfully crafted black tornado against the dimming orange sunset. But a few thousand feet away, directly in their flight path, a community of 3,500 homes would soon stand.

That was the plan at least, until Halloween.

In an effort to protect both bats and would-be residents, The Nature Conservancy of Texas and Bat Conservation International — with help from other groups, including the Department of Defense and the City of San Antonio — purchased the land for $20.5 million, providing a buffer between the bats and the encroaching urban sprawl.

The 1,521-acre plot called Crescent Hills is roughly 30 minutes northwest of San Antonio. The area is adjacent to the famous bat cave and lies within the Edwards Aquifer recharge zone. The aquifer provides drinking water for roughly 2 million central Texans, including the entire city of San Antonio. The property is also home to the federally endangered golden-cheeked warblers, birds known locally as golden finches of Texas that nest in the area.