The idea of linking the two sides of Istanbul underwater was first dreamt of by Sultan Abdul Mecit 150 years ago.

Now that Ottoman dream is finally being realised.

But the modern version of that vision has hit a historical stumbling block.

Istanbul archaeologists have uncovered a 4th-Century port at the site where engineers plan to build a 21st-Century railway hub. The Marmaray project cannot even begin work in the area until excavations are complete.

Out in the middle of the Straits, marine engineers are now working day and night to compensate in advance for any delays. Boring beneath the waves, they are preparing the ground for the deepest tunnel of its kind.

"We are strengthening the soil by injecting concrete into the seabed so we can place the tubes easily and take measures to counter earthquakes in the area," an engineer explains, shouting above the din of an enormous drill working non-stop behind him.

Parts of the Marmaray tunnel will eventually run just 6km (3.7 miles) from the active North Anatolian fault line.

"This is the best way to link the European and Asian sides of Istanbul. There is no space for a third bridge," he argues.

The Istanbul authorities hope the Marmaray project will ease congestion in a sprawling and increasingly overcrowded city. The rail link should carry well over a million passengers a day, significantly reducing boat traffic on the Bosphorus and car congestion on land.

But the railway was supposed to be running by 2010. Now its managers are not so sure.

Yenikapi on the European side of the city was selected to house a state-of-the-art train station. But when shanty homes were cleared from the site, archaeologists uncovered treasures beneath of a kind never before discovered here.

Just a few metres below ground, they found an ancient port of Constantinople - named in historical records as the Eleutherios harbour, one of the busiest of Byzantium.

"We've found 43m of the pier so far," chief archaeologist Metin Gokcay explains, pointing to a line of wooden stakes emerging from a green pool of water. He says the Marmaray site has yielded the most exciting finds of his long career.

"We believe there used to be a platform on those sticks - down there is where the horses were unloaded."

"We've also found lots of things that tell us about the daily life of the city in the 4th Century," Mr Gokcay enthuses, standing close to a tunnel he suspects was an ancient escape route.

"We found leather sandals, for example, with strings through the toes and around a thousand candle-holders and hairbrushes. I've done many digs in Istanbul, but there are many things here I've never seen before."

As well as the stone remains of the harbour itself, Mr Gokcay and his team have uncovered perfectly preserved ancient anchors and lengths of rope. Dozens of men are still scrubbing the mud of centuries from hundreds of crates of artefacts, for assessment.

But perhaps the site's most treasured find is stored beneath a large protective tent.

Inside, dozens of jets spray water to preserve a wooden boat that is more 1,000 years old. Its base, about 10m long, was discovered intact beneath what was once the sea.

The dig has uncovered eight boats in total - another first for Istanbul - and archaeologists believe there are more to come.

It's a dream discovery for them, but a nightmare for the Marmaray management.

"It's true I lose sleep over this. I worry we won't make it on time," admits Marmaray Project Manager Haluk Ozmen. He says the dig is only delaying work at the Yenikapi site for now, but warns it will soon affect the entire project.

"The dig is the only thing that can delay the Marmaray project. That's why we're working 24 hours a day to meet our deadline. Everything is in the hands of the archaeologists now."

Engrossed in their task, those archaeologists refuse to be rushed by commercial concerns. Their work was scheduled to finish four months ago, but they now reject all talk of deadlines.

"The Marmaray team cannot spread their cement or tunnel any deeper here until we finish," states a determined Mr Gokcay. "They have to wait for us. And I will continue my work here until the last artefact made by human hands is found. It's impossible to accept anything else."

In addition to the Eleutherios harbour, the dig teams have exposed a long section of the city wall from the days of Constantine I - the first time the wall has ever been uncovered.

At a site as rich as this, there's no telling what else could turn up.