California has been hit by heavy rain and flooding since the end of 2022, resulting in the formation of sinkholes in some locations, and with more wet weather ahead for the state, it's possible more could open up.

Sinkholes are gaping holes that form when the ground below the land surface cannot support the material above. They can vary in size from a few feet to hundreds of feet across, and from a few feet to tens of feet deep.

Sinkholes typically form very slowly, to the extent that the change in the ground is barely noticeable. But sometimes the collapse of the ground can happen suddenly.

Heavy rain and flooding can increase the likelihood of these sinkholes forming, as evidenced by the recent weather in California.

This aerial view captured on January 10, 2023, shows two cars swallowed by a large sinkhole that formed in the Chatsworth neighborhood of Los Angeles, California during a day of relentless rain. Storms have caused widespread flooding in the state since the end of 2022. ROBYN BECK/AFP via Getty Images

The state has been struck by several "atmospheric river" events since late December that dumped huge amounts of rain and snow, causing widespread destruction and the deaths of at least 18 people.

Amid the wet conditions, the formation of a number of sinkholes was reported in the state. One large sinkhole opened up in the middle of a street in the Chatsworth neighborhood of Los Angeles during a day of relentless rain, swallowing two cars in the process, the Associated Press reported. The drivers subsequently had to be rescued by firefighters.

Another damaged more than a dozen homes in the rural community of Orcutt, in Santa Barbara County. And authorities had to close a highway in Half Moon Bay—a small coastal city south of San Francisco—due to the formation of a sinkhole.

But how does this phenomenon occur?

"Sinkholes form in regions where there are many subterranean cavities and the bedrock experiences structural failure," Steve Brenner, a professor in the Department of Geography and Environment at Bar Ilan University, Israel, told Newsweek.

This structural failure may occur in so-called "karst" regions where the bedrock consists of soluble rocks, such as limestone.

It may also occur due to human activities where there is a significant withdrawal of groundwater due to extensive development and building or where there are extensive man-made cavities, such as tunnels or mines.

Another view of the sinkhole that opened up in the Chatsworth neighborhood of Los Angeles, California. The motorists whose vehicles were swallowed by the sinkhole had to be rescued by emergency crews.ROBYN BECK/AFP via Getty Images

"When these regions are suddenly inundated with massive amounts of water from torrential rainfall or flooding, for example from storms or from broken water mains, the soil layer above the cavities will become saturated, dissolution of the already fractured bedrock may accelerate, and the significant additional weight above the weakened bedrock or cavities will lead to collapse and opening of the sinkhole," Brenner said.

Timothy Bechtel, a professor of geosciences at Franklin & Marshall College in Lancaster, Pennsylvania, told Newsweek there is a "very strong and well-established" link between heavy rain or flooding and the formation of sinkholes.

"It is not just a possibility, it is a certainty in the right settings," Bechtel said.

The enabling factor when it comes to sinkhole formation are environments where there are open spaces or voids underground, with soil above them.

"This is inherently meta-stable—that is, gravity would like to pull the soil down into the void. Water is always the causative factor. Infiltrating water can find cracks that lead to the void, and the water will carry soils or soil 'fines'—the smallest particles—with it into the void," Bechtel said.

"The initial void can be a cave in karst areas," he continued. "The karst caves form over millions of years, but the overlying soils can be flushed into them in a matter of hours during intense rain storms or flooding. In urbanized areas, the void may be a sewer pipe, or subway tunnel, or any bit of infrastructure that represents an opening underground."

Sinkholes are relatively uncommon in California. The most damage from these events tends to occur in states like Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania, data from the U.S. Geological Survey shows. But with meteorologists forecasting more flooding and rain for California over the next few days, there is every chance more sinkholes will form in the state.

The National Weather Service (NWS) Los Angeles said Thursday that a series of approaching storms to affect southwestern California over the next five days could bring occasional "moderate to heavy" rain. Meanwhile, NWS Sacramento said Friday that "multiple rounds of moderate to heavy rain and mountain snow will bring renewed flooding concerns and dangerous mountain travel impacts over the next few days" to northern parts of the state.

Sinkholes pose a number of risks to people and property, sometimes resulting in injuries and death.

"All kinds of bad things happen when the earth opens up beneath you," Bechtel said. "Buildings are damaged, utilities rupture, people fall in—and are sometimes not recovered."

Vehicles and even entire buildings have been documented falling into larger sinkholes.

Particularly powerful storms such as hurricanes can significantly increase the risk of sinkhole formation in regions of the United States that are prone to such events.

"The link between hurricanes and sinkholes will be most noticeable in areas affected by hurricanes that have sinkhole-favorable subterranean conditions, such as the state of Florida, which has large regions with limestone bedrock," Brenner said.

A sinkhole risk map for Florida published in 2017 showed that the central and north-central parts of the state have the most favorable conditions for sinkhole formation. Following hurricane Irma in September 2017, for example, more than 400 new sinkholes were reported across the state.

"There are some indications it may also be happening after hurricane Ian which hit in September of 2022," Brenner said.