The earth let out a threatening, thunder-like rumble in Puebla, Mexico right before it cracked open on May 30 — swallowing up over 1.5 acres of farmland. The owner’s house sat teetering near the void’s edge, temporarily spared by the hungry earth.
It’s not caused by aliens or agricultural subterfuge: Officials on the scene have determined that this new landscape feature is a sinkhole, a kind of indentation in the earth that can appear suddenly to swallow up homes, cars, and even people. They can sometimes occur naturally, while others might appear due to human developments.
Sinkholes carry a certain air of mystery with them thanks to their subterranean origins, but the science behind them is actually pretty simple — albeit, difficult to control.
What is a sinkhole?
They may look like portals deep into the earth’s core or into the underworld, but sinkholes are actually just places where the earth has given way due to either soil erosion beneath it or a sudden loss of water. The resulting indentations, or sinkholes, can be as small as a softball or massive like the one that opened up in Puebla.
While sinkholes can pop up across the world, there are certain types of terrain that are more susceptible than others. Namely, a type of terrain called “karst” — riddled with soft, porous rock like gypsum or limestone — is particularly susceptible to developing sinkholes. Gypsum and limestone are easily dissolved by groundwater and can leave delicate caverns in their wake.
In the U.S., about 20 percent of the land is at risk for sinkholes thanks to this kind of terrain and consistently wet conditions. Florida, which is notoriously swampy, is a particular hot spot for sinkholes.
In 2017 alone, the state experienced 32 sinkholes. And in 2013, a Florida sinkhole opened up under a man’s home while he was sleeping and plunged him 20 feet below into the ground. His body was never recovered.
Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania are also top targets for sinkhole development.
What causes sinkholes?
While the end result may look the same, there are several different ways a sinkhole can form. One of the most devasting, and newsworthy, is something called a cover-collapse sinkhole.
These kinds of sinkholes can suddenly collapse within the span of a few hours and are likely the kind of sinkhole that bloomed recently in Puebla. But don’t let the sudden collapse fool you. That’s just a sign that there were structural problems under these spots for much longer.
Cover-collapse sinkholes form when the top part of the ground (called the “overburden”) begins to trickle into a cavity deep underground. As this loose sediment — largely made of clay — begins to deplete, it leaves a hole in its place. This creates a kind of soil bridge that can support less and less weight until it eventually collapses quickly in the chasm below.
Another common type of natural sinkhole is a cover-subsidence sinkhole that creates a slower kind of collapse when overburden slowly trickles down into a cavity below the ground, creating an indentation in the spot over time.
Sinkholes can also develop from manmade activity, like construction, the disrupts the natural flow of water in an area. Rerouting groundwater, for example, for something like an aquifer can disrupt the delicate balance between the overburden and water (where pressure from the water holds up the ground above it) and create a collapse.
This is likely part of the story of what happened in Puebla, Newsweek reports.
“We think that it might be a combination of two factors: the softening of the field, the whole area was being cultivated, as well as the extraction of groundwater, which softens the subsoil,” said Puebla's environmental secretary, Beatriz Manrique.
Are sinkholes dangerous?
Even though the sudden appearance of sinkholes may be scary, they’re not always inherently dangerous. That said, if sinkholes open up under heavily populated areas or essential services (like sewers) that can be a recipe for not only injury but potential water contamination as well.
It can be difficult to know exactly where — or when — a sinkhole will strike, but getting geological surveys done often (especially in sinkhole-prone areas like Florida) can be one way to keep on top of it. In addition to ground penetrating radar (GPR) for sinkhole risk analysis, surveyors may also use Interferometric Synthetic Aperture Radar (InSAR) to monitor slow-growing subsidence sinkholes.
And if you do find yourself starring into the dark void of a sinkhole — try to stay away from the edge.