One of the biggest challenges in tunnel construction, as well as one of the biggest incentives for creating long tunnels using conveyor rollers, is to create underwater links between different islands and different nations.
Because of this, the third longest railway tunnel in the world, and the longest underwater tunnel of any type is one that many people in Great Britain and France are incredibly familiar with.
The Channel Tunnel, often also known as the Eurotunnel, travels across the Strait of Dover between Folkstone in Kent and Coquelles near Calais in France.
Much like many other proposed and completed tunnels in the UK, its origins came from somewhat hypothetical attempts starting in 1802.
Albert Mathieu-Favier was the first to propose a tunnel bored under the English Channel designed for horse-drawn travel, the locomotive still decades away from common use.
His design involved creating a two-level tunnel, one for transport and the other for flows of groundwater, with the potential to create an artificial island in the middle that could be used for changing horses and literally catching your breath,
This plan did not get very far, but Aimé Thomé de Gamond would perform several hydrographical surveys in 1839 to determine the feasibility of such a tunnel, reporting to Napoleon III in 1856 after decades of political unrest that for 170m Francs (£7m) they could mine a railway tunnel from Calais to Folkstone.
After the creation of the Suez Canal, arguably the most important man-made transport link in history, the biggest issue was less a matter of practicality but more a matter of politics.
It was believed at the time that it would be a national security issue, and this concern would only truly stop in the 1950s when the rise of passenger airlines made the sea defence question irrelevant.
The project finally started receiving proposals in 1995, with the only rail-based submission being approved in 1986. It finally opened in 1994.
