Underground salt mine. 19th century artwork of the salt mine at Wieliczka, Poland. This is one of the oldest salt mines in the world, started in the 13th century and 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. Here, miners are seen descending a shaft (upper left), leading horses down ramps (centre left), praying at a church (upper right), and excavating salt (bottom). In 1978, the mine became a UNESCO World Heritage Site. Artwork taken from Grands Hommes et Grands Faits de l'Industrie (Great Men and Great Facts of Industry), France, circa 1880.