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Here's what it's like to enter Japan's Seikan Tunnel, which connects two islands —Hokkaido and Honshu—and is one of the world's longest underwater tunnels. The Seikan expanse is close to 34 miles long and almost half of that distance, 14.5 miles, is underwater:

There is something unnerving about that moment when the outside light recedes, and all that's left is the lamps flashing by. If you were on that train, some part of the brain would register that it was under a crushing amount of weight and that only really good engineering was keeping it from being crushed.

In 1986, when the Seikan Tunnel was opened but trains had not yet started carrying passengers through it, the New York Times heralded the accomplishment, writing that "a new era of underwater tunneling is under way." Now, they said, instead of carefully excavating underwater tunnels in painfully small increments, as they had since the early 1800s, tunnel builders might use giant tunnel boring machines to make the process go faster, or they might simply excavate trenches underwater and drop in lengths of tunnel segments.

The idea of tunneling under bodies of water instead of building bridges over them is intuitive enough that some of today's most iconic undersea tunnels were conceived of in the 19th century. But it's only in the last few decades that technologies like the ones the Times described has allowed these tunnels to exist.

The world's newest long underwater tunnel, which last year connected Europe and Asia under the Bosphorus Strait, was first thought up by an Ottoman sultan in 1860. A French engineer first had the idea for the Chunnel in 1802. Napoleon was into it; the British were not and would not be until Margaret Thatcher approved the Chunnel's construction, just around the time the Seikan Tunnel was being finished. The Chunnel opened in 1994—it's shorter, overall, that the Seikan Tunnel, but more of it is underwater.

Still, creating a long underwater tunnel is challenging enough that many of the world's tunnel dreams have yet to be made real. In 1986, the Times mentioned ideas for tunnels between Morocco and Spain, Japan and South Korea, the Canadian mainland and Prince Edward Island, and among Denmark's islands. Those first two ideas are still being tossed around—every few years, it's decided these projects are too technologically, economically or politically unfeasible and every few years, the idea comes up again.

But some of these things do get built. Norway's currently building the longest undersea tunnel for cars in the world. China seems very serious about building a 76-mile tunnel—as long as the Seikan and Chunnel tunnels together—under the Bohai Sea.

The most ambitious, unrealized vision for an underwater tunnel, though, is of a trans-Atlantic tunnel connecting New York and London. Technologically, it's an unlikely prospect, but it's fun enough to imagine that in 1935, Maurice Elvey, who made more films that any other British director, turned the idea into a movie:

Spoiler: They make it, but not before encountering an undersea volcano and having to tunnel around it. The real tunnel is still just an dream—a "high speed pipe dream," as the BBC put it.

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