Who ever thought that civil engineering could be sexy? Or an underground so shiveringly mysterious - a trip into the dark unknown? Has no one ever dug a tunnel through a city centre or near ancient buildings before? Or drilled a borehole or driven a subway through a watery bed?

For good or ill, they're doing just that every day around the world.

"A dip through a city centre is very common. You can see it in Singapore, Atlanta and in Washington too, I think," says Peter McMahon, general manager of London's Bakerloo Line. "Most of London's underground is actually above ground. So it's very normal to be on the surface and to dive underground where property prices get prohibitively high or traffic disruption is not on. . ."

Keith Beattie is general manager of London Underground's chief engineers group. "We're just coming to the end - I hope - of the Jubilee Line project and it couldn't have been run through more influential areas," he says. "In England, you can hardly get more sensitive buildings than Big Ben or the Houses of Parliament, so we've been tunnelling beneath and adjacent to some very, very old and very expensive buildings. But modern tunnelling technology and ground treatment can cope with most geological conditions these days."

Would he be alarmed at the thought of drilling into what Irish geologist Dr Adrian Philips describes as "very large water mains" created by ancient river bed channels? "Not at all. There are all kinds of things you can do. By pumping nitrogen into the ground at low temperatures, you actually freeze the water in the ground and turn it into a sort of rock which you can then tunnel through while it's still frozen. Then you build a waterproof tunnel and let the ground thaw out. . ."

But a lack of 100 per cent waterproofing hardly spells disaster either. Drips will always be evident in London's more elderly brick tunnels - some as old as 160 years.

"My railway runs along the river Thames and you always get some drips in the underground," says Peter McMahon. "Two and a half million gallons of water leak into the London Underground every day. You just have to treat it and pump it away. But even if all the pumps were to collapse, the tunnels probably wouldn't fill up with water."

The crucial boreholes seem less of a challenge too. Before a tunnel is even dreamt of, the proposed route will have to be peppered with boreholes about 25 metres apart (a massive disruption in themselves), the objective being to discover what lies in store below ground. But what if the tunnel route is beneath an ancient building where such drilling seems impossible?

"You drill straight down in the normal way in the road adjacent to the building," says Keith Beattie.

Modern engineering is also throwing up some clever ways of building the tunnels themselves. One of these is the NATM (New Austrian Tunnelling Method) whereby excavation, metal reinforcement and concrete lining - squirted on - are all done simultaneously, a metre at a time. This technique however, relies on having "reasonably good quality ground like London clay", says Keith Beattie.

"We didn't use it on the eastern end of the Jubilee line extension where we've got bad ground. There we had to use the traditional tunnelling method of boring a hole and erecting the lining. . ." Which is probably how it will have to be done in Dublin too? "Yes, it does sound like there are a lot of parallels between the eastern end of the Jubilee and the problems you've got in Dublin."

And there's the rub. Clearly, there are few surprises that civil engineering is unable to cope with these days, but there is no way out of the resulting mess: "An underground is enormously disruptive during construction," says Mr Beattie, "and nobody can hide that fact".

"Going underground for a tunnel is the easy bit," says Peter McMahon. "It's when you have to build an underground station. . . Digging that makes a big hole in the ground. Your ramp, or entry point, probably will be the length of Stephen's Green - maybe bigger. You've got to have escalators, platforms, etc. You're going to have a whacking great hole while building it. . . How do you plan to get the soil out? Where are all those lorries going to go in the city?"

Mr Beattie's team sent a lot of the muck down the Thames in barges. And of course, there's the cost, the dark unknowable at the heart of the Minister's "plus".

Undergrounds cost around 100 times more than surface systems. The notorious Channel Tunnel overrun (from £468 million to a soaraway £9.6 billion), often dragged up as the definitive example of tunnel trauma, is discounted as atypical by both men - albeit with chilling resonances for the Irish project.

"I don't believe the overruns had anything to do with the technical side," says Peter McMahon. "It's much more likely that if they had put forward their true figures to begin with, everybody would have walked away. So you come up with the lowest figure possible to get a start. . ."

Mr Beattie agrees that the job was undercosted - but for different reasons. "I think they underestimated the sheer volume of work."

The question of safety is something that received little attention this week but, according to Mr McMahon, can pose a far greater difficulty than the sexier civil engineering side.

"The biggest difficulty is putting in the signalling system. A whole regime of logic is required to make sure it's safe."

As for any possible Wood Quay scenarios in the course of construction, this too is familiar territory to both men. "Before you start, you should be prepared and agree how to deal with it, if something does turn up like the first Viking brewery. Anticipate delays like that so that if people are running around looking for legal advice, it will have minimal impact," says Mr McMahon.

Even when the whole thing is up and running, security - both general and personal - is another issue that has to be taken seriously.

"Once you go underground, you have to have a whole different safety regime," says Mr McMahon. "The three big risks are flooding, fire and terrorism. These are very, very infrequent of course but you have to have a quantified risk assessment. Every room in every London Underground station is inspected every hour against terrorism. Video cameras are pointed on every nook and cranny. There has to be staff presence. Clear plans have to be designed, worked out and practised regularly on how to get people out as quickly as possible if need be."

So, given a choice, would these men plump for overground or underground?

KEITH BEATTIE sees the arguments for and against. "Though you think you've cracked most geological problems, there will always be surprises underground." But the strong environmental lobby against train noise on his patch still nudges him toward the underground option - "once you stick it beneath the ground, it's out of sight and out of mind."

Mr McMahon, however, has no doubts at all. The son of London-based Irish parents and married to an Irishwoman, he knows Dublin well.

"Dublin is traffic madness now. I'd say it's worse than London. Light rail is a double win: you get light rail and fewer cars. If you jam the roads up, cars disappear."

He cites Manchester as a shining example of a modern transport system: "It runs on surface streets, it has cut car use tremendously and opened up sleepy, under-used suburbs. Dublin needs to bring some hard analysis to bear on this. An underground is going to be bloody expensive, will need a load of safety factors and then you have a dirty great underground station.

"Go above ground on the other hand, and you have a surface station, you have much simpler safety systems - and you haven't the cost of digging the hole underground."