For decades, scientists have marveled at the towering mounds some termites construct and wondered how they work. Although it’s now widely believed the 1- to 2-meter-high structures (seen above) help with ventilation—exchanging stale air for fresh in the insect’s hidden nest—the mechanics behind such a system have remained a puzzle. Now, by using thermal imagery and installing tiny air-flow sensors in about two dozen termite (Odontotermes obesus) mounds, scientists think they have solved the mystery. Their investigation revealed that the mounds act like an “external lung,” harnessing the change in temperature as day becomes night to drive ventilation. Here’s how it works: Inside the hill is a large central chimney connected to a system of conduits located in the mound’s thin, flutelike buttresses. During the day, the air in the thin buttresses warms more quickly than the air in the insulated chimney. As a result, the warm air rises, whereas the cooler, chimney air sinks—creating a closed convection cell that drives circulation, not external pressure from wind as had been hypothesized. At night, however, the ventilation system reverses, as the air in the buttresses cools quickly, falling to a temperature below that of the central chimney. The reversal in air flow, in turn, expels the carbon dioxide–rich air—a result of the termites’ metabolism—that builds up in the subterranean nest over the course of the day, the researchers report online this week in the Proceedings of the National Academy of Sciences. Although the scientists only examined one termite species, the ventilation system is likely the same in others with similar mounds. Such insight could inspire developments in passive architecture, which seeks to eliminate the need for active heating and cooling systems in buildings using strategies such as natural ventilation and efficient insulation, the scientists say.