 The miners thereupon flooded the rock with large masses of a boiling water solution and vinegar thrown with force to disintegrate it internally.At that point, the rock became sufficiently brittle to be successfully attacked by hammers and picks that were forcefully inserted and acted in the cracks.The path to the Monte Viso Tunnel starts at Pian del Re, where the river Po is born, at about 2,020 metres of altitude, taking the "V16" path leading to cross country crossing.