Tunnel du Mortier, at an elevation of 1.370m (4,494ft) above the sea level, is an abandoned high mountain tunnel with a length of 500m, situated in the Rhone-Alpes region and belongs to the Alps (France). It cuts straight through the mountain below the plateau de la Moliére, just as the Gouffre Berger does, but in a more linear form.
The road has been closed for years. It’s no longer usable after a severe landslide not far from the tunnel that happened not so many years after the road had been opened in connection with the Olympics in Grenoble. The damage to the road and the situation was so serious that no attempt has ever been made to restore the road. On April, 20th, 1992, a breakdown, estimated in 20,000 m3, occurred below the top of the Buffe, collapsing the road and creating a zone of instability. This collapse happened just 1 km far from another important collapse in 1971 (50 000 m3). After 1992, the road was totally closed to traffic at the Buffe point.
There is a plaque by the North entrance of the tunnel saying it was constructed for the 68 Winter Olympic Games, but according to the Autrans web site there's been a tunnel since 1911 and 1968 was only an upgrade. The inauguration of this upgrade was on September, 30th, done by Louis Verger.
There are 2 possible routes to reach this tunnel. By the south side, the road is totally passable. It’s an asphalted road, narrow, but in good conditions. But on the north side, the road has collapsed completely and it’s impassable to cars. You can ride through the tunnel, but there is a rock slide that has cut off the road further down the climb. You need to be very careful to cross, but it can be done and you can then descend into the Vercors. The tunnel is passable, but you need to bring a light. It’s pretty long and obviously completely dark. The surface is smooth and devoid of obstacles and manholes. There are a few smaller rocks towards the Isere end, but by then it's light enough to see.
To arrive to the tunnel, starting from Veurey, the ascent is 19 km long. Over this distance, the elevation gain is 1199 meters. The average percentage is 6.3 %. And starting from Saint Quentin sur Isère, the ascent is 20 km long. Over this distance, the elevation gain is 1199 meters. The average percentage is 6 %. The tunnel was built for two full lanes of traffic. It has no lights, and it's long enough, 500 meters, but lots of light pouring in from the ends on a clear day. The first maybe 100 meters are easy enough but the entire middle section can be pretty dark. Try to ride on the center as long as you don't look up at the far end. The light at the end of the tunnel will wreck your vision. There’s a real deathly risk on this tunnel.