The Engaña Tunnel is a long railway tunnel of almost 17 km. (6,976 meters exactly, measured starting from the South mouth) which is located between the municipalities of Vega de Pas (Cantabria) and Pedrosa de Valdeporres (Burgos), with a width of 8 meters designed to accommodate two roads and a height of 6, 5 m., and takes its name from the nearby Engaña River.
It is part of the Santander-Mediterranean railway project that sought to link the Cantabrian Sea with the Mediterranean and which was never used. A project that was projected to be completed in 4 and a half years was extended for 17 years, being abandoned at the Yera Station, which was the last thing that was done before its abandonment.
Currently it is almost completely collapsed inside (around km 2,800) and since 1999 it has not been possible to drive through it or cross it. At the northern mouth you can see that the water infiltrates from the river seeps through its walls and falls even in a cascade. The construction materials were quite poor and it has degraded very quickly.