Permafrost by permafrost. Of this area slightly more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, and a little less than 30 percent by sporadic permafrost. Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost". Permafrost typically forms in any climate where the mean annual air temperature is less than the
Historic Silver Mine in Tarnowskie Góry Adit, part of the Deep Adit Fryderyk, was the first to be exploited for tourism. In 1957, it was opened as a 600 metre long boat tour. The Historic silver mine opened in 1976. The visit in Historic Silver Mines starts in a interactive museum where tourists get the knowledge of methods of silver, lead and zinc ore extraction as well as underground mine drainage. Then tourists go underground. The trail is 1,749 meters long; 270 meters are travelled by boats. Outside the buildings of Historic Silver Mine there is an Open Air Steam Engine Museum. The new attraction of