The Blegny mine, located near Liege, was one of the four most important coal mines in Belgium in the past. It was closed in 1980 as the last mine and is now a UNESCO World Heritage Site. For a detailed description see. Wikipedia. The image was taken underground and shows the course of a secured tunnel.
<a href="//commons.wikimedia.org/wiki/User:Horst_J._Meuter" title="User:Horst J. Meuter">Horst J. Meuter</a>
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<span class="int-own-work" lang="en">Own work</span>