Flood defences. Miner in the Wieliczka Salt Mine adjusting a cement injection drill used seal a wall and make it watertight. Flooding is a constant problem here, and the lowest level of the mine is entirely flooded. 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.