A tunnel by definition is an underground passage. While there is still a lot of controvery as to what constitutes a tunnel, generally the ratio of the length to the width of the passage of a tunnel should be at least 2 to 1. A tunnel should also be totally enclosed on all sides, except for the openings.
The Seikan tunnel, which is built in Japan is the longest rail tunnel found in the world. It is in total 53.9 km (33.4 miles) long and has 23.3 km (14.5 miles) under the sea. Once the Gotthard Base tunnel is fully constructed in Switzerland, it will be the longest underground tunnel in the world. The world's longest road tunnel is the Lærdal tunnel in Norway. The exact origins of tunnel building is a debatable topic. The Babylonians, for example had built a tunnel (circa 2180 B.C.) under the Euphrates by means of what we now call the "cut-and-cover" method. The Egyptians constructed tunnels as entrances to tombs.
The Seikan tunnel, which is built in Japan is the longest rail tunnel found in the world. It is in total 53.9 km (33.4 miles) long and has 23.3 km (14.5 miles) under the sea. Once the Gotthard Base tunnel is fully constructed in Switzerland, it will be the longest underground tunnel in the world. The world's longest road tunnel is the Lærdal tunnel in Norway. The exact origins of tunnel building is a debatable topic. The Babylonians, for example had built a tunnel (circa 2180 B.C.) under the Euphrates by means of what we now call the "cut-and-cover" method. The Egyptians constructed tunnels as entrances to tombs.