Methane burning. Artwork titled 'The Penitents' showing miners using lit flaming torches to burn off methane gas in the Wieliczka Salt Mine. This artwork is by the artist Alphonse Dlugosz, who in 1961 became the first curator and director of the museum at the mine. 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.