This is a tunnel of the WW1.We did 4 flights before to get the ground floor.It's a long underground passage, well hidden and camouflaged in a wood, not far from Cortina d'Ampezzo, very close to P.sso Tre Croci.
You need a torch also in the daylight,if alone,you'd be brave ;) too!
All this area houses many sites of the ww1.Enjoy them!