There is a small rock arch high on the northest side of the Barranc Sacanyar, atop the rocks of the Costera del Pubil. Geologists explain that these arches are the remnants of tunnels formed by underground rivers. Imagine that the mountains were once many times higher than they are now. Being limestone they eroded easily and underground rivers developed. Over millions of years the mountains themselves were worn away. The harder rocks remaining the longest. Thus the arch is all that is left of a tunnel which once ran deep beneath the mountain.