Currently under construction, the Fehmarn Belt Tunnel linking Germany with Denmark is set to become
the world's longest pre-fabricated undersea road and rail tunnel when it's completed in 2029. But another subsea tunnel also being built, this one in China and scheduled for completion within a decade or so, looks likely to break that record. Longer, deeper, and with more capacity for traffic is what subsea tunnels are all about in the 21st century. But how exactly have these
pioneering examples of civil engineering evolved over the last 180 years or so, and what underwater tunnel is presently the longest and deepest of them all?
Click through this gallery and submerge yourself in the history of undersea tunnels.