As a vision of the future Dublin's port tunnel project is both grimy and sleek, writes Edward Power.

It has been hailed as the most ambitious feat of engineering undertaken in the history of the State - a 4.5. km subterranean highway that will link Dublin Port to the M1 motorway and, it is hoped, alleviate the capital's chronic gridlock.

Two years ahead of completion date, however, the port tunnel is weirdly reminiscent of the underground lair of a cash-strapped James Bond villain that has been unceremoniously smothered in several hundred tonnes of rubbery gloop.

At close quarters, the scale of the endeavour is overwhelming but so too are the biblical quantities of dust, muck and well... more muck.

As it inches towards midway point, the project is already living up to its hyperbolic billing: this is really a breathtaking proposition; a simultaneously sleek and profoundly grimy vision of the future.

It has also proved a relentless source of controversy. Subsidence which earlier this month forced the closure of commuter rail lines in north Dublin was blamed on tunnelling work.

Accusations that a subcontractor had imported cheap labour from Africa, precipitated a fraught, though ultimately short-lived, face off with the unions. There have been claims that the structure will be too low to accommodate tall goods vehicles. It is hardly surprising then that Mr Matt O'Reilly, spokesman for Nishimatsu Mowlem Irishenco, the Japanese-British-Irish consortium contracted to build the tunnel, grows exasperated when conversation turns to Dublin's continued ambivalence towards the project.

He blames the city's unfamiliarity with engineering works of this scale. But it is the very size of the undertaking that may prove its most enduring contribution to the capital, he argues.

"In Ireland we're not used to works of this level. In that respect the tunnel is breaking the mould. It is a step into the unknown.

"People have said that it would never come to pass - well it will, and hopefully it will lay the groundwork for other endeavours of similar scope and ambition."

Ambition is undeniably a quality possessed in abundance by the designers of the tunnel, ranked one of the five largest underground engineering projects currently in development.

Approximately 20m deep, the completed structure will consist of twin 11m in diameter tubes, each supporting a 4.6 km two-lane highway.

Some 20,000 vehicles are predicted to traverse the tunnel each day in its opening year, rising to 31,000 in 2018.

The journey will last seven minutes - a fraction of the time it currently takes to reach the port from Dublin's outskirts.

But all this is in the future.

The tunnel is in its infancy, comprising two dust-wreathed ducts measuring several hundred and several dozen metres in length respectively.

The longer of the pair will stretch south from the tunnel access shaft at Whitehall, beneath Griffith Avenue through Fairview, emerging at the mouth of the River Liffey, near East Wall road.

The northbound tunnel is considerably shorter - it will eventually lead from Whitehall to the M1 Dublin-Belfast motorway at Coolock Lane interchange.

It was decided, after considerable deliberation and much debate, that two tunnel-boring machines (TBMs) should be employed to excavate the bulk (2.6km) of the route, with the remainder (1.9km) consisting of a pre-cast "cut and cover" structure (a cut and cover tunnel is created by excavating a trench from the surface down, constructing the tunnel inside the trench, and filling in the trench afterwards).

Nicknamed "Grainne" and "Megan" the TBMs are anything but feminine in aspect.

Clambering through the innards of Grainne, the bigger and more specialised of the two, as it chugs and grinds against the rock face, you are reminded sharply of the audacious character of the undertaking.

The apparatus generates ear-splitting volumes of noise. A conveyer belt noisily carries huge shards of stone to the surface, where they will be pounded into gravel and used to line roads.

A claustrophobic warmth provides an incongruous counterpoint to the omnipresent muck, an uncomfortable reminder that you stand 20m below the surface, equivalent to the deepest levels of the London Underground.

In Grainne's control room a Donegal flag adorns the wall - the county is renowned for producing tunnellers of the highest quality.

Grainne works around the clock - although she will slow to a two-shift pattern when she passes beneath Griffith Avenue in early 2003.

Rejecting claims that drilling will inflict unacceptable levels of noise disturbance upon those living above the route, Mr O'Reilly says tunnelling generates low-level vibrations which might cause anxiety were they to arrive unannounced but which will impose minimum disruption on forewarned householders.

Aside from its obvious benefits - easing the congestion which is slowly suffocating Dublin, stymieing the development of the port and reducing the city's attractiveness to foreign investors - the tunnel offers far-reaching long-term benefits, says Mr O'Reilly.

With the Luas light rail project edging towards completion and gridlock approaching critical mass, it is apparent that the capital stands on the brink of a period of profound infrastructural change.

Construction of an underground rail network could prove essential as Dublin attempts to shrug off decades of chaotic traffic planning - or, more often than not, no planning at all.

Should the tunnel prove a success, it will offer irrefutable proof that no matter how significant the short-term disruption, large-scale underground engineering is a viable proposition, paving the way for the creation of a metro, Mr O'Reilly believes.

"At the moment Dublin just doesn't work. Traffic is a nightmare, something you would expect of a third world city.

"The tunnel will prove to people that we can take affirmative action, that we can build large-scale engineering structures, the equal of anywhere in the world.

"Many of the engineers working on this project are, like me, Irish people who went abroad for experience and are now coming home.

"We want to do something for the country, we think it should have a modern, efficient transport system. We are confident the tunnel is the first step in this direction."