Originally published July 26, 2016.

With more than $3 billion worth of commercial property within its borders, including the massive King of Prussia Mall, Upper Merion Township in Montgomery County is one of the region's richest in terms of property wealth.

But despite all that prosperity, the township long has had an underlying problem: sinkholes.

The story of how they came to be is about 500 million years old, but the problems they cause are here and now.

During the King of Prussia Mall expansion that begin in 2014, the ground continually caved in. To date, it has happened 35 times.

"That's the thing about sinkholes - you don't know where or when they are going to appear, and you don't know the size," said Robert Cottone, president and CEO of IMC Construction, which managed the site's construction.

King of Prussia sits on what geologists call karst, a landscape of dissolvable limestone prone to sinkholes. While the rock is almost a half-billion years old, experts say more development could exacerbate the sinkhole problem. But that's no sure thing.

"It's not a guarantee that something's going to happen, and it could happen," said geologist Laura Toran of Temple University's department of earth and environmental science. "Don't assume that all limestone is going to have a sinkhole. It doesn't."

In 2004, Upper Merion mapped more than 200 reported sinkholes, and there are many more, including one that developed last month. Toran advised against halting development, but advised caution.

"To not build on that rock type would wipe out too much land," Toran said.

Less than a mile away from the mall, construction is underway for the next huge King of Prussia development. In the $100 million town center - an outdoor shopping and restaurant venue with mixed residential options - real estate developer JBG Cos. hopes to create Upper Merion's downtown.

The 125-acre site used to be an 18-hole golf course. It now features one million square feet of commercial space, including a now-open Ulta Beauty; seven restaurants; more than 3,000 apartments; 132 town houses; and a grassy town square.

A large portion of that area is on land the township has labeled highly probable for developing sinkholes.

Every state has some karst areas; almost the entire state of Florida is on the landform. A 2015 study by the U.S. Geological Survey estimated that the nation spends a minimum of $304 million on sinkhole damage a year, and listed Pennsylvania as one of the most sinkhole-prone states, with an average yearly cost of $1.8 million.

Karst expert George Veni, executive director of the National Cave and Karst Research Institute, said he has found a link between sinkholes and development. In a region of Florida that he studied, sinkholes were much more frequent on developed land with urban infrastructure than on natural, undeveloped land.

Cottone said that given the vetted process for planning, engineering, building, and remediating for all construction projects, including both the mall expansion and new town center, he had no fears about public safety. He said geotechnical surveys of the land were used before construction to precisely plan preventative measures against sinkholes for buildings' foundations and water system.

Cottone said national or regional standards wouldn't make sense.

"You can't have a one-size-fits-all approach," he said. "You're customizing engineering and design and architecture. . . . That allows a safety net of professionalism for each individual site, but not a scripted solution that may not be applicable on every site."

Sinkholes form when any kind of circulating water washes out the sediment that has filled void space from dissolved limestone, said state geologist Gale Blackmer. That causes the land on top to cave in.

"Sinkholes tend to appear after changes in the groundwater level," Blackmer said. "Times of drought, when the water goes down and maybe the water was supporting the soil, or it can also happen in times of excessive rainfall."

But a new development with a pond, pipes, or sewage can have a similar effect, introducing new water into the bedrock from leaks or runoff, said Kevin McCartney, who worked on the civil engineering for the new town center. He said the project was most careful implementing water features because of the buried landscape.

"The introduction of water, that is the only thing you have to prevent, water going into an area that wasn't seeing it before," said McCartney, assistant project manager from Bohler Engineering.