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.