From the quayside of the Danish town of Rødbyhavn, the Baltic sea sparkling in the sunshine, Germany is so close you can see it – a cluster of wind turbines on the horizon.

The German town of Puttgarden is just 11 miles away, but getting there involves either a 45-minute ferry journey – plus time for embarking and disembarking – or a six-hour drive taking in much of Denmark.

But under the water, construction of an alternative future is well under way, in the shape of the world’s longest immersed tunnel – the term for a tunnel built elsewhere and then sunk into place. When the Fehmarnbelt tunnel opens in 2029, people will be able to cross the stretch of water in as little as 7 minutes by train and 10 minutes by car.

The tunnel will far eclipse the current record holder for road and rail, the 2.5-mile Drogden tunnel, which with the Øresund bridge connects Denmark and Sweden.

The new tunnel is also seen as the final stage of a three-part infrastructure project – alongside the Øresund bridge and the Great Belt Bridge that spans the Danish islands of Zealand and Funen – connecting Denmark, and the Nordics as a whole, with the rest of Europe.

Once the tunnel is completed, it will be possible to transport goods, including steel, by rail from as far north as the top of Norway to central and southern Europe. It will considerably cut the train journey time between Copenhagen and Hamburg from 4.5 to 2.5 hours.

At the national level it symbolises the culmination of a transformation of the Danish psyche over the past quarter of a century, from a relatively isolated peninsula and collection of islands to an international hub.

“It’s part of this link which Europe is trying to make between northern Norway and southern Europe, which started when we built the Øresund bridge between Copenhagen and Malmö,” said Mikkel Hemmingsen, CEO of state-owned company Sund & Bælt. “To finish the link, we have to build this connection.”

Unlike a lot of tunnels that run underwater – including the Channel tunnel – which have been constructed using boring machines that dig through the ground, sections of the Fehmarnbelt tunnel are being built above ground and submerged on to the sea floor. Hemmingsen said this approach was more efficient and better for the environment.

The Guardian was granted exclusive access to the first 120 metres of the tunnel last week. Ships passed on the water above the tunnel entrance as workers welded the outer walls. Close to the site, long lines of lorries lined up to board the ferry over to Germany.

The tunnel, which will comprise 90 217-metre-long components that weigh 73,500 tonnes each, is being constructed at a factory that was built specifically for the project.

The completed sections will be floated on the water and submerged before being connected together on the seabed, which reaches a depth of about 30 metres.

Hemmingsen said making the tunnel bore more of a resemblance to building electronic components than traditional construction

“What we’re doing right now is not so much a normal construction project, where you have a lot of people running around like small ants in a limited space. It’s more like building iPhones actually.”

There are six production lines – designed to be “optimised and lean” – each producing a different element of the tunnel.

Hemmingsen compared piecing about 900 small elements together to form 90 larger elements of the tunnel to “using Lego blocks”.

“If you fail at one element you can actually replace it,” he said. “Once it’s done it looks very simple, but from an engineering perspective there are a lot of details that have to be in place before you can start.”

Although the construction time of the €7bn (£5.94bn) project is relatively short – work started in 2021 – it has been a long time in the making. Engineers have been putting samples of concrete into the water in the area for the past 15 years to see how different mixes react to the salt levels. The project is paid for by a state-backed loan that will be repaid using tolls, and has received more than €1bn from the European Commission.

Anders Wede, the construction area manager, grew up nearby before going away and returning to work on the tunnel. He hopes the factory – which will be put to use on different projects once the tunnel is finished – could help bring down the region’s relatively high unemployment rate.

“The region here has suffered a lot the last 20-30 years,” Wede said. “They [the local authorities] really see this as the golden opportunity to have a new chance, and maybe some companies see potential in the area,” he said. “So they are really crossing their fingers.”

The area will benefit from reduced journey times to Germany, he added, and more tourists are expected to come from Europe through Denmark and into Sweden. “Before this was the corner of Europe,” he said. “Now it’s the centre of Denmark going into Europe.”