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.