Mine workers, Wieliczka Salt Mine. These miners work in shifts, as the mine needs to be manned 24 hours a day. When one shift finishes, another one starts. 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.