Beijing’s Fengtai District grabbed headlines in April when the ground beneath Shiliuzhuang Lu opened up suddenly and swallowed a passing truck. This isn’t the first time sinkholes have appeared in the capital. In 2007, and again in 2009, sinkholes opened up near Dawang Bridge in the CBD, leaving residents to wonder if the great Fengtai sinkhole of 2011 might not be the last time the capital’s ground drops out from under it.

News reports attributed the incident to subway tunnels being dug in the area. It occurred near the Songjiazhuang Station, the terminus of Line 5 and the beginning of the new Yizhuang line.

“It could have resulted from multiple causes,” said Beijing engineer Wei Jinglian, quoted in the Global Times. “Water leakage from old pipes underground may have softened the soil, or there has been too much pressure frequently on the same section of road. A hole 20 meters deep above a subway tunnel—that’s awfully rare in Beijing.”

Not all sinkholes are caused by construction. They frequently appear on karst landscapes, which are formed by layers of soluble bedrock, often limestone or dolomite. These landscapes contain turbulent aquifers that erupt on the surface as natural springs. In December, Xinhua reported that over two-thirds of Beijing’s water comes from underground sources, and that the city has been drawing on karst groundwater 1 km or deeper below the ground since 2004. These sources, says Xinhua, “were originally set aside for use only in times of war or emergency.”

Beijing’s water shortages were exacerbated by this year’s almost snowless winter. Without precipitation to replenish underground aquifers, overuse can cause the water table to drop, weakening brittle patches of ground. Though there’s no evidence now to link this specific sinkhole to water shortages, it’s a dramatic reminder that we’d do best to err on the side of conservation.