A futuristic vision of a hypersonic underwater tunnel may revolutionize travel between the UK and the US, with trains zooming at 3,000mph - similar to the £16trillion Transatlantic Tunnel.

Hyperloop One, a Los Angeles-based startup developing the technology for the Hyperloop high-speed transportation system, is interested in building a Hyperloop that can travel underwater.

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.