An antiquated technique that generations of Bostonians used to add thousands of buildable acres to their city is coming back to haunt their city’s current residents.

The ground beneath some homes is literally rotting away and it isn’t cheap to fix, according to the Wall Street Journal.

Around 5,000 acres of Boston is built atop soil and gravel landfill that was once Massachusetts Bay. That land isn’t solid enough to build foundations on, so up until the 1920s, builders drove massive wood pilings down to solid ground and built foundations on top of them.

Pilings can remain viable for centuries when submerged in groundwater, but when exposed to air they can rapidly rot away beneath foundations. Public works projects and construction over the years has caused groundwater levels to drop in some parts of the city, exposing pilings.

There are roughly 6,000 buildings constructed on top of such pilings, including many in the city’s most expensive and sought-after neighborhoods. Remediation — replacing exposed wood with steel — alone can cost upwards of $200,000, not accounting for any further repair work to an actual building.

Many sellers don’t want to know if their homes have damaged pilings and because the housing market is so hot in the city, some buyers are willing to roll the dice and skip the expensive and time-consuming testing needed to investigate potential issues.

“Few people investigate it—they don’t want to know,” Compass agent Doug Miller told the Journal. When working with buyers, “if I see settling, it’s definitely a question that I ask, but I’ve never had anyone answer with anything other than ‘I don’t know.’” [WSJ] — Dennis Lynch