The Provost of Trinity College, Dr Tom Mitchell, has promoted so much new construction during his term of office that he might be regarded as having an edifice complex. But Trinity is expanding at such a pace that it needs a constant supply of new facilities just to keep pace with academic and other demands.

Going underground is the college's latest stratagem. Assuming that Dublin Corporation grants planning permission later this month, a huge hole 14 metres (46 ft) deep will be excavated at the north-eastern extremity of the campus for a subterranean sports centre and five new lecture theatres.

The entire railway viaduct curving through the site is to be demolished and replaced by a modern structure - a task which the structural engineers, Ove Arup and Partners, say could be done in 56 hours - and the new viaduct will incorporate large glazed roof lights over the swimmingpool and sports hall below.

Designed by international architects Pei Cobb Freed, whose most acclaimed tour de force was the remaking of the Louvre in Paris, in association with Henry J Lyons and Partners, the presence of the underground complex would be indicated by a truncated glazed cone on the surface, rising to a height of 10 metres.

This would be set in a mostly hard-landscaped area which will serve as a north-eastern entrance to the Trinity campus, consciously opening it up to the city. The plan includes a new set of gates at the corner of Pearse Street and Westland Row, and the enclosed pedestrian overbridge there is to be replaced by a subterranean link.

What this will connect with is still an open question. Egged on by CIE and developer Bernard McNamara, Trinity was considering a much larger scheme, also by Pei Cobb Freed, under which its relatively recent Goldsmith Hall student residence would have been demolished to redevelop Pearse Station, in Westland Row.

THAT plan, which included a substantial high-rise element, was put on hold after An Bord Pleanala's decision last September to refuse planning permission for Skidmore Owings and Merrill's "towers of light" scheme for George's Quay and will not be resurrected until after the corporation completes its high buildings study.

In the meantime, Trinity decided to proceed with the projects planned for the area straddling the railway viaduct on the west side of Westland Row - the underground sports centre, lecture theatres and other facilities, and a six-storey block to house IITAC, its new Institute of Information Technology and Advanced Computing.

The latter has been designed by Brian O'Halloran and Associates and Cullen Payne Architects as a major extension to their Materials Science building which won an architectural competition in 1996. The first two phases of the project are already under construction and will be linked by an atrium to the third phase now proposed.

IITAC is described as "a vital part of the national educational strategy for computer sciences". The Government regards it as a "must have" project to put the country in the forefront of information technology by turning out suitably qualified graduates; it is also intended to meet the demands of Trinity's fastest-growing department.

The college is funding the sports centre from capitation fees. According to Martin Donnelly, the HJL partner in charge, it will be one of the most up-to-date facilities in Ireland, including a 25-metre, eight-lane Olympic "short course" swimming-pool as well as an adjoining "warm-up" pool and a large gallery for spectators.

The centre will also provide a multi-purpose sports hall, capable of being divided in three, with bleacher-type seating, as well as an ancillary hall, a projectile hall, sports clinic, squash and racquet ball courts, a hydrotherapy spa, fitness centre, wet and dry changing rooms and lavatories - all underground.

Concourse areas would link the sports centre with the five new lecture theatres (four with a capacity of 150 and the fifth seating up to 550 students, making it Trinity's largest) and the new IITAC and Materials Science buildings above ground. The scheme also includes some highly-serviced laboratory accommodation.

Apart from the swimming-pool and the main sports hall, both of which are fullheight, the subterranean space is arranged on three main levels, with the first floor providing a major circulation route linking the existing O'Reilly Institute and new lecture theatre concourse to the atrium between the IITAC and Materials Science buildings.

As in the Louvre, this concourse would be daylit by a glazed pavilion at ground level - though, in contrast with the diamond-sharp pyramid in the Cour Napoleon, Pei Cobb Freed opted for a truncated cone on the Trinity site, probably because it is bounded by two curves (the railway viaduct and the railings on the street corner).

Prior to the US architects' involvement, Robinson Keefe and Devane had designed a six-storey building for the corner site, restituting the damage done by road-widening. Mr Donnelly admits that the latest scheme, which would leave the site largely open, is the antithesis of what was previously proposed.

Though the corner site suffers from a weak containment - some of the surrounding buildings are only three storeys high - the architects argue that it must be looked at in the larger context of the college and its future expansion. "We see this as having a character not unlike the front gate on College Green," they say. Trinity's frontage to Pearse Street is so introverted and unsuccessful that Harry Cobb, the Pei Cobb Freed partner in charge of the project, concluded that a grand gesture was needed to open it up. "You have to change your mind-set", according to Mr Donnelly. "I'm not saying it takes an American architect to do it, but he did."

COMPARED with the existing carpark and railway viaduct, he believes that the glazed cone will have "a certain sculptural quality" - a phrase much in vogue these days - though the idea of constructing an entire building of 100,000 sq ft underground (plus another 50,000 sq ft of lecture theatres) is certainly unusual.

"It would have been very difficult to elevate these two spaces (the swimming-pool and sports hall) to a street," says Mr Donnelly. Each of them would have risen at least 40 ft, perhaps with largely blank facades - like Scott Tallon Walker's sports hall by the west side of the railway viaduct, which doesn't address the street at all.

The future use of this building, which rigidly follows the grid of the college, is still being considered. It will become redundant following completion of the underground sports centre and may receive similar treatment to Aras an Phiarsaigh - built out to the back of the footpath and converted to some other use.

Blank walls are surely not the only option. One can imagine a swimming-pool, for example, as a tank in a box with circulation spaces behind a glazed facade. But a decision has been taken to put the entire sports centre underground and have it operational in time for the Special Olympics in 2003.

Trinity is also thinking long-term outside the confines of the college. It already owns the former An Post site on Pearse Street, just north of the railway station, and the IDA Enterprise Centre further east, beyond Macken Street.

The sports centre was earmarked for the former An Post site, now used as an overflow car-park for the college, but even this proved tight for what would have been a partly underground complex, also designed by Henry J Lyons and Partners. Building underground within the college's boundary frees up the other site for future development - something that is no doubt already exercising the mind of Trinity's energetic director of buildings, Tim Cooper, and, of course, the Provost.