Could we ever build a transatlantic tunnel?

The vision sounds irresistible: step onto a train in New York, and emerge 54 minutes later in London, having traveled through a tunnel beneath the Atlantic Ocean. This kind of travel is described in some recent proposals. But is a trans-Atlantic tunnel really possible or the stuff of science fiction?

The short answer: It's probably not possible with current technology.

First of all, the 54-minute journey would require vacuum trains traveling at 5,000 mph (8,000 km/h) — technology that doesn't exist yet. With conventional rail speeds, the trip would take around 15 hours, making it slower than an 8-hour flight.

Currently, the world's longest undersea section of a tunnel belongs to the Channel Tunnel, which has a 23.5-mile (37.9 kilometers) underwater section connecting England and France. Construction on the tunnel, nicknamed the Chunnel, took six years, 13,000 workers, and 4.65 billion pounds in 1994 (12 billion pounds, or $16 billion today).

Depending on where you build the tunnel, it can cost much more — both in time and money. The Hudson Tunnel Project, for example, is an effort to construct a 9-mile (14 km) rail tunnel between New York and New Jersey that's predicted to take 12 years and cost $16 billion.

"It's one project, but it's really 10 different projects within one, each of which is almost a mega project in and of itself," Steve Sigmund, chief of public outreach for the Gateway Development Commission, the organization behind the Hudson Tunnel Project, told Live Science.

A trans-Atlantic tunnel, of course, would be considerably longer.

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The most popular dream of a trans-Atlantic tunnel would be between London and New York, which would stretch around 3,400 miles (5,500 km). For a tunnel like that, "there's going to be several challenges," Bill Grose, a tunnel expert and Institution of Civil Engineers fellow, told Live Science.

The first challenge would be the logistics of building it. " One would have to solve how to ventilate a tunnel like that, how to supply power to a tunnel boring machine, and how you would get the workers to site," Grose said.

The time it would take to transport workers from one end of the tunnel to the halfway point would be impractical, Grose said, so the project would require a fully autonomous tunnel boring machine — a device that hasn't been invented yet on a scale that could burrow an underwater tunnel for human vehicles.

And that's before you account for the power demands. For even a 6-mile-long (10 km) tunnel, a typical tunnel boring machine requires about the same amount of power as that of a small town, Grose said.

Plus, tunnel boring machines are slow. For a tunnel that spans the shortest distance across the Atlantic — Gambia to Brazil, around 1,600 miles (2,575 km) — "that would probably take something like 500 years at the current speed of the tunnel boring machine," Grose said. "You'd really want something that works 50 times faster than modern technology."

There's also the challenge of water pressure. "You have to be really careful about the amount of pressure that exists, both in terms of digging the boring machines in the tunnel themselves, but also … making sure people are safe," Sigmund said. "And that's just 1 mile across the Hudson. So multiply that by a thousand, [and] you're going to run into some very serious issues." Things like leaks, gushing water and tunnel collapse have led to financial losses and death in past undersea tunnel projects.

The world record for water pressure faced by a tunnel boring machine is 15 bars, or 15 times atmospheric pressure at sea level, around 500 feet (150 meters) below the water's surface. At its deepest, the Atlantic Ocean is more than 27,000 feet (8,000 m) deep, which is 800 bars of pressure.

"So you can imagine that while you would make every endeavor to get so deep that you didn't encounter any water, if you did, it would be mega catastrophic," Grose said.

Finally, there's the problem of funding such an enormous project. "Construction, materials, time, labor, people planning — that's really the major pieces of it," Sigmund said, describing what drives tunnel costs even for relatively short projects.

Given the enormous cost and catastrophic risk of a single leak, funding such a project would be nearly impossible.

"At the moment, I would say that the challenges are fairly insurmountable," Grose said. "There are some things that need to be invented."