War against the Austro-Hungarian Empire was declared May 24, 1915. This put Tyrol on the front line, which passed through some of the highest mountains in the Alps. The ensuing front became known as the War in ice and snow, as troops occupied the highest mountains and glaciers all year long. Twelve metres (40 feet) of snow were common during the winter of 1915-16, and tens of thousands of soldiers disappeared in avalanches. The remains of these soldiers are still being uncovered today. The Italian Alpini, their Austrian counterparts (Kaiserjäger, Standschützen and Landesschützen), and the German Alpenkorps occupied every hill and mountain top. They began carving extensive fortifications and military quarters, even drilling tunnels inside the mountains and deep into glaciers, like at Marmolada. Hundreds of troops would drag guns over mountains up to 3,890 m (12,760 ft). Streets, cable cars, mountain railways and walkways through the steepest of walls were built. The first to occupy higher ground became almost impossible to dislodge, so both sides turned to drilling tunnels under mountain peaks, filling them up with explosives, then detonating the entire mountain, including its defenders, such as Col di Lana, Monte Pasubio, Lagazuoi, etc. Climbing and skiing became essential skills for the troops of both sides and soon Ski Battalions and Special Climbing units were formed.