The IJ Klopper is an underwater tunnel built in Amsterdam. This tunnel is meant for cyclists. This tunnel has a spiral system with a 50-meter wide drum on both ends. The tunnel can be used also from pedestrians, but they can access just using separate lanes. Pedestrians can have access to the tunnel via stairs or elevators. This innovative tunnel can also be three times cheaper than a bridge over its lifetime.
Link: IJ KLOPPER