Building the “Cave of Dreams”

Note: This is the third part in a blog series on The Nature Conservancy’s new artificial bat cave, an experiment to help stop the spread of white-nose syndrome, a fungal disease killing millions of bats. Read the previous blogs.

Cory Holliday frequently evokes Field of Dreams when talking about the artificial bat cave: “Build it and they will come.”

It’s true that bats readily use human-made habitats, from bat houses to mine shafts to attics.

Still, when the goal is to protect large numbers of bats from a deadly disease – perhaps, even save them from extinction – you have to rely on more than chance. You have to consider what will make the artificial bat cave a healthy, attractive place to spend the winter. You have to make it better than the real thing.

Just digging a hole in the ground won’t cut it.

Holliday, the cave and karst program director for The Nature Conservancy in Tennessee, considered every detail available on bat natural history and hibernation habits to ensure that the bats will indeed come to the artificial cave being constructed near Clarksville, Tennessee — the first of its kind.

So what are the components of a “cave of dreams?”

Keep it Close

This one is pretty simple: Build it close to a natural cave that is home to the endangered gray bat. Bellamy Cave has 260,000 gray bats, it’s a Conservancy project and there was a logical place to place the cave 100 yards away.

But even when this ideal location was chosen, construction couldn’t begin immediately. That’s because Bellamy Cave is also a maternity colony, meaning gray bats have babies there. Young bats—called pups—often fly haphazardly when they leave the nest.

There was a real risk of them darting out of the cave and crashing into construction equipment, defeating the purpose of the artificial cave.

So, construction began as soon as they left the cave, in late summer. But that meant there was a rush to complete the project before bats begin arriving to hibernate, which can be as early as mid-October.

While the cave could hold as many as 200,000 bats, a more realistic goal is 10,000 to 15,000.

Keep it Cool and Moist

The most critical factor in designing the cave was temperature. Gray bats hibernate in caves that are in the 50 degree range. Any warmer, and the bats will avoid it.

The bat cave is designed to be a cold air trap. It is situated at the bottom of the hill, so cold air naturally sinks. Still, the cave had to have ventilation to ensure a constant temperature. The Conservancy turned to refrigeration engineers.

Basically, the cave has tubes that function as chimneys. They capture and trap the cold air, which flows evenly through the cave.

But there’s one added wrinkle: One part of the cave is actually designed to stay warmer, to attract species of bats that need or prefer a less chilly hibernating spot.

The bat cave is constructed of concrete. When earth was cleared away and the structure installed this summer, it was exposed to the sun. This heated the cave up, and it is taking a while to cool down.

“Our biggest battle right now is lowering this temperature,” says Holliday. “It has to drop a bit before it’s ready for bats.”

To accomplish this, Holliday has portable air conditioning units cranking in the cave.

The artificial bat cave also must remain at a relatively constant 85% humidity. A tube supplies water into a small, shallow pool that will keep the humidity level high throughout the winter.

Bats do arouse from their torpor throughout the winter. When they do, they often seek a drink of water, so the pool doubles for this purpose.

Keep it Clean

The other key component of the cave is ensuring that it can be easily cleaned — thus killing white-nose fungus — each spring.

The walls have to be designed for easy disinfecting, but they also have to be places bats would actually roost. Bats hang “upside down” on the ceiling of caves. But they can’t just cling to a flat surface. A variety of surfaces are used in the artificial cave – remember, it’s the first of its kind so experimentation is vital – to see what bats most prefer. In some areas, the surface merely has texture; in others, netting provides an extra foothold for bats.

The bats naturally move out of the cave in the spring, moving to maternity caves or smaller caves around the forest. When they do, staff will enter the cave and kill the fungus.

A variety of cleaning methods will be tried, from high-pressure water treatments to household disinfectants.

Keep on Trying

As with any experiment, it’s hard to know how exactly it will turn out. That’s why Conservancy staff members are considering every possible way to make this a welcoming place for bats.

In a few weeks, bat calls will be broadcast from the cave entrance in the hopes of drawing them to the opening. At one point, Holliday even considered using bat guano to replicate the smells of a real bat cave, but experts suggested this wouldn’t make a difference, and would risk introducing the fungus.

“We’re trying everything we can think of to make the habitat suitable and draw the bats,” says Holliday. “Not everything is a viable option, but there is a lot going on here. It may look like a concrete bunker, but it represents the best thinking out there on how to mitigate the white-nose syndrome disaster.”

Thermal cameras will be installed so that the bats can be monitored, without disturbance, throughout the winter. (I’ll be following the latest news on the artificial cave’s results, so check back for the latest information on how the bats are faring).

The final component is to ensure this project is replicable. The hope is the cave becomes a key tool in fighting white-nose syndrome.

“Hopefully, this is a pilot project,” says Holliday. “If it works, you could build more of them. You could build them larger. You could probably do it for less cost, because you’d know what exactly works and what doesn’t.

“We’re also buying time,” he continues. “The more we can slow down the spread of the disease, the more time we have for research that can make a difference.”

(Photo: Construction at the artificial bat cave near Clarksville, Tennessee. Credit: Paul Kingsbury/TNC.)


If you believe in the work we’re doing, please lend a hand.


  1. Awesome! But I can’t help but think that concrete box + sunshine = oven. Shouldn’t you put this a bit further below the surface?

  2. Hi Joshua, I’m the Communications Manager for The Nature Conservancy in Tennessee. Good question. We consulted experts not only on bats but also on cooling large structures. They led us to make sure the cave would be buried under at least 4 feet of earth and constructed so that it would achieve a cooling air flow in its hillside location. We had to temporarily use air conditioners inside to lower the initial temperatures. But now we’re letting Mother Nature do its work. You’d be surprised how much cooler it is down there below ground now.

Add a Comment