Denmark Germany tunnel: Travel time from Copenhagen to Hamburg to halve as new record-breaking tunnel built - when will it open?

Running for 18km (11 miles), the Fehmarnbelt will be the world's longest pre-fabricated road and rail tunnel. It will see segments of the tunnel placed on top of the seafloor, and then joined together.

The project's main construction site is located at the northern entrance to the tunnel, on the coast of Lolland island in the south east of Denmark. The facility spans more than 500 hectares (1,235 acres), and includes a harbour and a factory that is manufacturing the tunnel sections, which are called "elements".

To make each 217m (712ft) long and 42m wide element reinforced steel is cast with concrete. Most underwater tunnels - including the 50km Channel Tunnel between the UK and France – burrow through bedrock beneath the seafloor. Here instead, 90 individual elements will be linked up, piece by piece, like Lego bricks.

"We are breaking records with this project," says Mr Vincentsen. "Immersed tunnels have been built before, but never on this scale."

With a price-tag around €7.4bn ($8.1bn; £6.3bn) the scheme has mostly been financed by Denmark, with €1.3bn from the European Commission. It's among the region's largest-ever infrastructure projects, and part of a wider EU plan to strengthen travel links across the continent while reducing flying.

Once completed, the journey between Rødbyhavn in southern Denmark and Puttgarten in northern Germany, will take just 10 minutes by car, or seven minutes by train, replacing a 45-minute ferry voyage. Bypassing western Denmark, the new rail route will also halve travel times between Copenhagen and Hamburg from five to 2.5 hours, and provide a "greener" shortcut for freight and passengers.

Denmark and Germany signed an agreement to build the tunnel back in 2008, but the scheme was delayed by opposition from ferry operators and German conservation groups concerned about the ecological impact. When the tunnel opens in 2029, Femern estimates that more than 100 trains and 12,000 cars will use it each day. According to plans, revenues collected from toll fees will repay the state-backed loans that were taken out to finance the construction, and Mr Vincentsen calculates that will take around four decades.