Could a Russia-US rail tunnel be built?
Russian officials have backed the idea of a rail tunnel linking Russia and the US.
It would run under the Bering Strait for 105km (65 miles) - twice the length of the UK-France Channel Tunnel.
The tunnel itself has been estimated to cost $10-12bn and to take 10-15 years to build.
But an additional 4,000km (2,485 miles) of new track would be needed to link it to Russia's rail network, plus another 2,000km (1,243 miles) to connect to existing services on the US side.