This former railway tunnel through the hills in northern Spain is now Europe's longest bike tunnel.

The nearly half-mile tunnel links two neighborhoods in San Sebastian, which has a population of around 400,000 and is trying to increase bike commuting. The tunnel opened in 2009 at a cost of $3.7 million.

For visibility, the tunnel's walls were painted white and lighting was installed throughout. It also contains 29 security cameras, a guard, and is closed overnight.