In mining, it is called a tunnel when there is a horizontal passage from the surface of the earth into the interior of the mountain for the purpose of mining. The usually arched gallery entrance is referred to as the tunnel mouth hole in the miner's language. For the miner, this “gate into the interior of the mountain” was of enormous importance since it had also been the entrance of daylight into the darkness of the earth. Often, the tunnel mouth hole is also being connected with the history and development of Christmas Arches. This is due to the great appreciation of the light by those miners working in the dark those days. This was particularly evident when the miners gathered on "Mine Christmas Eve" for their Mettenschicht (an old German mining custom) in the hut houses.