The Fehmarn Belt Fixed Link project would create a new 11 miles tunnel under the Baltic Sea, which would be the longest undersea tunnel in the world. Check out the video in the attached link for 4 minute overview of how the construction will be done. http://www.citylab.com/commute/2015/03/the-tunnel-project-that-could-reshape-the-european-map/388652/?utm_source=SFTwitter