Tunnel - Wikipedia. For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel.
China wants to build a massive 8000-mile high-speed train line connecting China, Russia, Canada, and the United States via a 125-mile underwater tunnel. The route would begin in the country's northeast and extend through Siberia and across the Bering Straight into Alaska.