For years, hundreds of thousands of worker ants have survived in a tiny nuclear bunker buried in Poland by eating the “corpses of the[ir] imprisoned nestmates.” Now, researchers have released the tiny beings back to their “mother nest” in an attempt to understand the insects’ complex evolutionary history.

Back in 2013, researchers had initially intended to count hibernating bats that lived in the same bunker, which measures 2.3 meters high and 3 meters by 1.2 meters wide (7.5 feet high and 9.8 by 4 feet wide). Scattered along the floor were several hundred thousand – if not a million – worker ants (Formica polyctena) trapped in a confined space with no light, no obvious source of food, and a year-round average temperature of no more than 10°C. Worker ants typically live in vast forested areas and require a queen to reproduce, but despite lacking access to the outside world, this “colony” of ants was thriving.

But how did they get here and, more importantly, how were they thriving?

Writing in the Journal of Hymenoptera Research, the researchers determined that the ants’ mother nest was located just above the bunker and individuals would fall through a vent in the ceiling. Without access to other food, live ants would eat the dead bodies of their comrades as they piled up along the floor of the bunker.

In nature, similar behavior has been observed in times of food scarcity. This concept of “ant wars” both sets territory boundaries between neighboring ant colonies and adds an extra bit of protein during shortages. But never has the behavior been observed in this type of setting.

"The present case adds a dimension to the great adaptive ability of ants to marginal habitats and suboptimal conditions, as the key to understanding their unquestionable eco-evolutionary success,” wrote the authors.

To confirm that the nest above belonged to the bunker ants, researchers took about 100 individuals and placed them along the outskirts of the mother nest. No aggressive behavior was observed, indicating that the ants were of the same colony.

“After our previous study, we started to contemplate on possible means to help the imprisoned ants to find their way out of the bunker. In practice, the only way to free the ants from the bunker would be to enable their spontaneous return migration to the maternal nest through the ventilation pipe,” wrote the authors, adding that they were helped by the “serendipitous observation” of a wooden board leaning against the wall.

In the Spring of 2016, researchers decided to free the captive ants. They took a 3-meter-long (9.8-foot-long) boardwalk found within the bunker and made a ramp that led from the floor up to the ceiling vent.

“Soon after the boardwalk had been installed, single ants started to inspect it,” noted the authors. Each ant made their way to the escape route and within six months, the bunker was nearly empty. Ants continue to fall through the ventilation pipe, but the ramp helps them return home after their inevitable fall. The “monumental” ability of wood ants to maintain self-organization and work together even when they’re no longer in contact with their mother nest suggests great adaptability in “marginal habitats and suboptimal conditions,” helping us to understand their “unquestionable eco-evolutionary success.”