The city of Berezniki, in Russia’s Ural mountains, is slowly sinking into the earth. The city of more than 150,000 individuals was built directly on top of a potash mine, which was standard practice during Soviet times. After nearly a century of extraction, deep voids were left underneath the city. The ceilings of these huge underground caverns are supported only by walls and pillars of soluble salt. In 2006, when a freshwater spring began flowing into the mine some 720 to 1,500 feet below the surface, it dissolved the supporting pillars and the city came crashing down.

A significant part of the residential districts and enterprises of the city are affected by the sinkholes, of which there are several in the city. The largest of them nicknamed “The Grandfather” is nearly four hundred meters across and more than two hundred meters deep. It threatens to engulf the only rail line which leads to and from the potash mines. Berezniki produces around ten percent of the worlds potash, and the mines are the city’s biggest employer. Closure of the mines would be damaging to the local economy.

A range of high-tech instruments including video surveillance system, seismic sensors, regular surveys and satellite monitoring of the changes in altitude of roofs, sidewalks and streets, are now used in Berezniki to predict the formation of sinkholes.

Federal officials and company executives are debating whether to relocate the entire city to the opposite bank of the Kama River, where the bedrock is solid. But engineers have assured them that the era of sinkhole formation is over, and no new holes will open as much of mine is now flooded.

About 12,000 residents have already left Berezniki for stable grounds, but the rest who’ve decided to stay put will have to keep close watch.

All photos by Lana Sator