An undersea train tunnel between Finland and Estonia is a step closer to becoming a reality, with a protocol of intent to build set to be signed by both countries on April 26. At 103 kilometres long - twice the length of the Channel Tunnel - it will stretch under the Gulf of Finland to connect the airports in both capitals, Helsinki and Tallinn, with two train stations in between. Currently linked only by ferry, the project aims to bring the two cities together into a single metropolitan area. It's hoped the tunnel, which could be ready by December 2024, will provide economic growth for the countries.