Harpea's Cave
Harpea's Cave (from the Basque "the place under the rock") is a cave located in Estérençuby, in the Navarre commune, a few meters from the Franco-Spanish border. It is an example of an anticline, a convex fold of strata, the centre of which is occupied by the oldest geological layers.
Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).