The city of Berezniki, in Russias Ural mountains, is slowly sinking into the earth.
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.
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 citys biggest employer.
Closure of the mines would be damaging to the local economy.
