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Tunnel sheilds are used to hold it up.
There are many possible answers to that such as a tunnel, an undersea tunnel, a great work of engineering, and international co-operation project and many others.
Chunnel (also called channel tunnel)
The Channel Tunnel is the undersea railway tunnel under the Dover Strait. It connects Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais in the north of France. The tunnel is owned and operated by a company called "Eurotunnel".
The Earth's largest railroad tunnel is the Seikan Tunnel in Japan, which is over 33 miles long. When the Gotthard base tunnel in Switzerland is opened in 2016, Seikan Tunnel will be the second longest.
the bakeyard tunnel is the name it 500 feet
Japan
It is the longest undersea tunnel in the world. It travels beneath the Tsugaru Strait-connecting Aomori Prefecture on the Japanese island of Honshū and the island of Hokkaidō-as part of the Kaikyo Line of Hokkaido Railway Company.
the eurostar in london
Mauna Loa, the world's largest active volcano, is found in the Hawaiian-Emperor seamount chain, a mostly undersea mountain range in the Pacific that is above sea level in Hawaii. The Hawaiian name "Mauna Loa" means "Long Mountain."
The world's longest canal tunnel is the Rove Tunnel in France. The Stad Ship Tunnel in Norway is shorter, wider, and deeper.
Undersea volcanoes that erupt.