There is a 13.8 kilometer long tunnel under the English Channel, the Eurotunnel. - Apparently.
The tunnel, also known as the Channel Tunnel, connects England and France, allowing trains to travel between the two countries. Construction began in 1986 and was completed in 1994, taking nearly 8 years to finish. The tunnel is maintained by Getlink, a French company, and is one of the longest undersea tunnels in the world.