Japan boasts the world's longest underwater tunnel, which stretches for more than miles under the seabed of the Tsugaru Strait, but it's now barely being used.

       Work on the Seikan Tunnel was completed in 1988, providing citizens with a direct route between the popular main islands of Honshu and Hokkaido, as per The Travel.com.

       Major cities, including the capital Tokyo, Osaka, and Kyoto, are on Honshu. The northern island of Hokkaido is a popular holiday hotspot with cooler temperatures and great skiing opportunities in Sapporo.

       The tunnel was designed exclusively for trains and is 33.46 miles long, of which around half is underwater.

       It was a remarkable feat of engineering which was also designed to withstand tsunamis and earthquakes, a neccessary precaution given that the country is located where four tectonic plates converge, and experiences some 1,500 quakes per year.

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       But the popularity of Seikan Tunnel is now said to be waning, only three decades after it opened to the public.

       It's thought this is due to flights between the islands now being quicker, as well as cheaper.

       Planning for Seikan came in the wake of World War Two, when the country had to resettle many returning people after their overseas territory was diminished.

       Originally, a fixed-link ferry was thought to be the main plan, but when five ferries sank in 1954, an idea for a train tunnel was put forward—especially as planned economic growth would have overwhelmed the ferry port, which couldn’t be expanded because of geographical features.

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       It was an enormous undertaking, with thousands of workers at the site at any one time. Tragically, 34 workers lost their lives, mostly in transportation accidents, The New York Times reported, three years before work wrapped up.

       Journey times from end to end are around 24 minutes, but the tunnel now sees far fewer passengers due to air travel, which was growing in popularity even as the project was being built.

       But with many travellers seeking more sustainable ways to move about the country, the tunnel may eventually see people returning to more environmentally friendly links between the Hokkaido and Honsu.

       The tunnel initially served slower commercial passenger trains, which travelled alongside regular freight tunnels transporting various goods.

       2016 saw the Shinkansen bullet train beginning services through the Seikan Tunnel for the first time, ending the slower passenger train services.

       But due to shockwave risks, bullet trains aren't allowed to travel at full speed when and freight train are passing through at the same time, which requires careful coordination and makes passenger routes less frequent, as per the outlet.