A group of about 20 people, including several wiggly children, sits quietly on cedar benches and limestone rocks as the sun sinks below the trees. Feet shuffle on the ground, someone coughs softly, a few people exchange whispers. The wind rustles the leaves, a plane passes overhead. Fifteen minutes pass. Only the promise of something amazing could keep us all this quiet.

Another sound, like running water or distant traffic, emanates from a dark slash in the ground, about 100 feet wide and shaped like an upside-down smile, some 50 yards away. The wings of millions of bats circling down in the cave make 'that sound,' which gradually grows louder. A few small shapes emerge from the slash followed by more, and then more still, until a swirl of rapidly moving gray shadows fills the air between us and the cave.

This is Bracken Cave, home to the world’s largest concentration of mammals — a colony of female Mexican free-tailed bats. They arrived here in March to raise their babies in the cave and emerge every night to spread as far as 60 miles in all directions to feed on insects throughout the summer.

This is the first year the public has been able to experience this phenomenon since Bat Conservation International first assumed ownership of the cave and 660-plus acres of wild Hill Country land surrounding it 20 years ago.

The organization conducts and supports science-based conservation efforts on behalf of bats around the world and hopes these tours will educate more people about the animals, the threats they face, and their importance to agriculture and native landscapes. Like the female Mexican free-tailed, many species of bats eat insects and are vital to plant pollination and the spreading of seeds.

Advance reservations for the bats' emergence are recommended, but can be made only a few weeks in advance, as check-in times vary depending on the time of sunset and the bats' recent behavior. It’s up to them, after all, when to head out for dinner.

The tour starts at Natural Bridge Caverns, where Fran Hutchins, BCI’s Bracken Cave Coordinator, gives an orientation on bats in general and Bracken Cave in particular. Then everyone piles into their cars and lines up behind Hutchins’ pick-up to caravan next door and down an unpaved road (pale caliche dust coats every vehicle afterward) to a parking area.

It’s a short walk from there to the edge of the sinkhole that contains the cave opening. This is where the group falls quiet (bats can hear extremely well) because if we make too much noise, the bats might wait until we leave to show themselves. That’s powerful motivation, even for the youngsters.

Soon, the growing tornado of bats bulges in one direction and then another as the wind buffets the tiny, 15-gram bats, at one point bringing them in right over our heads. A stream of bats heads east and southeast like smoke. They stay in close formation until past the gauntlet of owls and hawks that show up for an easy evening meal. The circling blur can continue up to four hours, Hutchins says.

Once the emergence is in full swing, it’s no longer necessary to remain silent, and Hutchins leads us to the other side of the sinkhole for a different view. The stream of bats has now become big enough to show up on Doppler radar, and he holds up a smart phone to show us the green splotch they form on the screen.

BCI may offer these tours again next year, pending results of studies on whether the presence of people this season had any effect on the colony. Just in case, plan to make it here before October, when the bats head back to Mexico. The whole experience takes two-and-a-half to three hours.

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Natural Bridge Caverns is 12 miles south of New Braunfels. Take exit 175 off I-35 and follow State Highway 3009. Tickets for the bat cave are $24.99 for adults or children. A combination ticket to tour Natural Bridge Caverns and see the bats is $39.98 adults, $31.98 children 3-11.