Humidity measurement, Wieliczka Salt Mine. Geologist installing a hygrometer to measure the humidity in the salt crystal cave at the Wieliczka Salt Mine. This cave, where large crystals of outstanding natural beauty form, has a permanent humidity of 80 percent. 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.