This tunnel is one of our favorites. We explored it for the first time in 2002. In the year of 2005 we went back and made a photoreport. Another revisit was made in the summer of 2006.
The tunnel crosses the mountain of St. Pierre. The tunnel has an opening in a marl-quarry. So there are two parts, one of 600 meters and a second one of 900 meters. The tunnel follows a slight bend to the left. It's difficult to see the trains coming. The second part of the tunnel is much more difficult to explore then the first one. It's a really nice tunnel and has the right length.
The tunnel du Geer was build by the Germans in the first world war. It was part of a fast connection between Germany and the Belgian cities of Antwerp and Liège. Nowadays it is a important freight line.