Europe, North: Figure 2. Calcite stalagmites in Grotte Père Noel, the abandoned high-level tunnel that lies parallel to the river passage in the Grottes de Han, in the Belgian Ardennes. (Photo by Tony Waltham)

DOI link for Europe, North: Figure 2. Calcite stalagmites in Grotte Père Noel, the abandoned high-level tunnel that lies parallel to the river passage in the Grottes de Han, in the Belgian Ardennes. (Photo by Tony Waltham)

Europe, North: Figure 2. Calcite stalagmites in Grotte Père Noel, the abandoned high-level tunnel that lies parallel to the river passage in the Grottes de Han, in the Belgian Ardennes. (Photo by Tony Waltham)