The Complex Engineering of Underground Ant Cities

These cities can house millions of ants and plunge 25 feet underground.
Fabienne Lang
The entrance of an ant nest.thamerpic/iStock

Look at an anthill and you'll just see a mound of dirt. But dig a little deeper, and you'll discover the architectural and engineering wonders of an underground ant city. These underground networks can house millions of ants, go as far down as 25 feet (7.6 meters), and last decades. 

So it's little surprise that scientists at Caltech decided to study ants' construction behaviors not only to better understand the lives of ants but to also see if we can learn a skill or two from these tiny insects and improve our underground digging abilities — something that could be useful for mining, subways or underground farming.

The study was published in the journal PNAS.

A look at how ants build their underground burrows

The Caltech team, led by Jose Andrade, the George W. Housner professor of civil and mechanical engineering at Caltech, closely observed ants' behaviors to understand how they were deciding to build their constructions. 

Asking Joe Parker, the assistant professor of biology and biological engineering at Caltech, to join their team, the scientists cultured ants and put them in little cups filled with soil so that they could then put them in an X-ray imager. 

The Complex Engineering of Underground Ant Cities
An ant city cast in a net next to an adult for scale. Source: Charles F. Badland/Caltech

The team was able to create 3D scans of the inner tunnels the ants had created in the cups, and create simulations of their progress. In doing so, Andrade and his team realized the ants followed a few patterns: 

    1. They were efficient: They dug their tunnels along the sides of the cups and as straight as possible, minimizing any extra work.

As Andrade stated "That makes sense because a straight line is the shortest path between two points. And with them taking advantage of the sides of the container, it shows that the ants are very efficient at what they do."

    2. They dug their tunnels as steeply as possible without exceeding the steepness, perfectly abiding by the laws of physics — otherwise, the tunnels would collapse and the ants would die.

A computer animation showing how ants dig tunnels in a cup of soil.
The Caltech team 3D scans from X-ray imaging the ants' cups of soil. Source: Caltech

And the third, and final, discovery the team made was something that could serve humans with their underground digging techniques:

   3. As they remove grains of soil, they naturally rearrange the force chains in the tunnel.

This means that as they work, they create little cocoons around themselves, which reinforce the existing walls of the tunnel, and which relieve pressure from the grains at the section of the tunnel where the ants are working, making it easier for the ants to safely remove them. 

"It's been a mystery in both engineering and in ant ecology how ants build these structures that persist for decades," Parker said. "It turns out that by removing grains in this pattern that we observed, the ants benefit from these circumferential force chains as they dig down."

One of the most fascinating discoveries of this study is that the ants don't appear to even be aware of their work methods, it just comes naturally to them.