A futuristic proposal for a train tunnel connecting New York and London just resurfaced, boasting a mind-boggling price tag of nearly $20 trillion. Dubbed the “Transatlantic Tunnel,” this highly ambitious project could all but revolutionize intercontinental travel.

The idea isn’t new. For decades now, engineers and visionaries have toyed with the concept. But technological and financial hurdles keep it firmly in the realm of science fiction. A flight between the two cities takes about eight hours. This tunnel could slash that to just one hour, thanks to cutting-edge vacuum tube technology, Newsweek reported.

Here’s how it works: by eliminating air resistance in a vacuum-sealed tunnel, pressurized trains could reach speeds exceeding 3,000 mph. It’s a design inspired by Elon Musk‘s 2013 hyperloop proposal, which envisioned ultra-fast, energy-efficient transport. The tech is already being tested in India and China for domestic high-speed rail, fueling optimism about its potential.

Still, this project’s scale is nothing less than staggering. 

The tunnel would span over 3,000 miles beneath the Atlantic Ocean. For comparison, the Channel Tunnel (or “Chunnel”) between Britain and France is just 23.5 miles long. That project, completed in 1994, cost $21 billion (adjusted for inflation) and took six years. That’s about 3.9 years per mile of tunnel. While the engineering doesn’t necessarily work out in such a measured tempo, in theory, at that pace, the Transatlantic Tunnel could take centuries to complete.

Other underwater transport projects showcase both the potential and the challenges. The Chunnel serves millions of passengers every year and is a critical link between the U.K. and mainland Europe. But its construction faced delays, cost overruns, and environmental concerns. An undersea tunnel between Denmark and Germany, set to open in 2029, offers another example of how such projects can reshape regional travel.

Despite its futuristic appeal, the Transatlantic Tunnel faces steep hurdles. No finalized design exists. Proposals range from tunnels on the ocean floor to floating structures tethered by cables. And with a price tag nearly equal to the U.S. national debt, the project would require unprecedented global cooperation.

Is the Transatlantic Tunnel a pipe dream? Yep…for now, anyway.