One of the big recurring mysteries of 2017 was the spate of unexplained booms which rocked the skies worldwide. In some cases, military activity was suspected, while in others meteorological anomalies or meteorite activity was thought to be the cause. Whatever the reasons, loud booming noises seem to be increasing in regularity lately, and answers are scant. Now, however, a series of unexplained booms across the American midwest is being blamed on a little understood phenomenon thought to occur deep within the ground. Have mystery booms gone underground?

Stay frosty, mystery booms.

Police dispatchers throughout central Indiana were baffled this week when residents across several counties called in to report loud booming noises powerful enough to shake homes. Some residents even thought heavy objects had fallen on their houses. “Like someone dropped bricks up there. Scared the bejesus out of me!” wrote Julie Justus of Irvington. It turns out, however, that the cause of these booms might lie under residents’ feet.

If we knew of all the terrifying phenomena that happened underground, we’d all live in balloon houses. Not a bad idea, actually…

According to meteorologists, the booms are likely an example of cryoseisms, or “frost quakes.” These seismic events occur when temperatures plunge below zero suddenly, causing groundwater in bedrock and the Earth’s crust to freeze and expand. That sudden expansion can cause bedrock to crack violently, causing the kinds of booms heard throughout Indiana this week. The same phenomenon was cited today as a possible explanation for booms heard in Cincinnati, Ohio.

Cryoseisms can sometimes be powerful enough to crack the earth.

Sure, the cryoseism theory sounds plausible in these cases as many U.S. states have seen record low temperatures lately, but that does little to explain other mystery booms throughout the past year. Are frost quakes merely a convenient explanation for something stranger or more secretive going on? Given recent military escalations and examples of secret aircraft testing and rocket launches, there’s no telling just what these booms could be. But sure, let’s go with frost quakes this week. That’ll keep people placated until the next round of booms.