Ground movement, Wieliczka Salt Mine. Mining engineer inspecting a crack in the ceiling of an underground gallery (passageway) in the Wieliczka Salt Mine. The markers embedded into the ceiling allow the engineers to track the widening of the crack over time due to ground movements. The salt mine at Wieliczka, Poland, is one of the oldest in the world. It was started in the 13th century and is still in use in the 21st century. By the 19th century, vast underground caverns had been excavated to depths of several hundred metres, including statues sculpted out of the salt deposits. In 1978, the mine became a UNESCO World Heritage Site.