Wooden support beams being inspected by a miner, Wieliczka Salt Mine. These wooden support beams were put in place in 1970, and some 30-40 years later have broken under the pressures caused by ground movements in the area. 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.