This cover from Poland depicts a se-tenant "Tourism"set issued in 2018. The stamp on the left shows a picture of the
Wieliczka Salt Mine. The areas around Wieliczka and and also around Bochnia contain deposits of rock salt, which has been mined since the 13th century. Hundreds of kilometres of tunnels in both mines also contain works of art, such as sculptures carved in salt and underground chapels. The mines illustrate the development of mining technologies in Europe. Wieliczka was first listed individually in 1978. Eleven years later, it was added to the list of World Heritage in Danger due to the threats posed to the sculptures by the humidity. Following a conservation program that saw the installation of efficient dehumidifying equipment in the mine, it was removed from the endangered list in 1998. A minor boundary modification took place in 2008. The Bochnia Mine was added in 2013.
