One-fifth of China’s urban housing stock has been bought up and left vacant, and it’s adding to the country’s housing woes.

China has a well-documented problem with ghost cities. Across the country, third- and fourth-tier cities are built on the peripheries of crowded metropolises like Shanghai and Beijing, with the promise of becoming economic hubs in their own right. Then they fail to attract businesses and residents, leaving a shell of a city that sits empty. That means millions of unsold housing units, rising property debt, and further fears that the inflated housing market has created a bubble that could soon burst.

But unsold properties aren’t the only problem. There are also tens of millions of units that have been bought—often by entrepreneurs and speculators who have no intention of living in them or renting them out. Some are bought as vacation homes or for single male family members, as it’s traditional for grooms to gift their brides an apartment. But in many cases, buyers hold on to them as investment property, with the hope of eventually selling them for a profit. Kaiji Chen, an economist at Emory University who studies China’s housing market, calls them “ghost apartments.”

These vacant units make up more than one-fifth of China’s entire urban housing stock, according to the latest nationwide housing survey by researchers at Southwestern University of Finance and Economics in Chengdu. That adds up to more than 65 million homes sitting empty nationwide—and they aren’t all in ghost cities. In fact, the survey found the issue to be the most severe in second- and third-tier cities. And if you look at mortgage data, it also includes first-tier cities like Beijing, Shanghai, and Shenzhen, too, says Chen, who wasn’t part of the housing survey.

A main driver for this dilemma is that speculators put faith behind the value of the land that their properties sit on.