The Great Bore in its time - a forgotten gem in ours

This Friday one of the wonders of the modern world will be damaged beyond measure by engineers and contractors working for London Underground (LU). Many people have used this wonder for the past 150 years, but few have known that they were the beneficiaries of one of the most extraordinary engineering achievements of the 19th century - the Thames Tunnel, linking Wapping and Rotherhithe and designed by Marc Brunel and his son Isambard Kingdom.

The twin-bore 1,250ft-long tunnel has long formed part of the Underground's East London Line. It was the first tunnel, certainly since antiquity, to have been driven under water. The methods used to construct it pioneered many of the techniques used in subsequent tunnels, notably the Channel tunnel, opened last year. It is now a scheduled Ancient Monument and a glorious, secret work of architectural engineering.

From Friday, LU will smother the Brunels' work in concrete; when the tunnel reopens in seven months, its character will have changed completely. The tunnel does not deserve this fate but as few people know anything about it, obscurity has aided its fall from grace.

The Thames Tunnel was begun in 1825 and not completed until 1843, at the cost of seven lives. Isambard Kingdom Brunel was very nearly the eighth when, in 1828, he narrowly escaped drowning after the Thames broke into the tunnel carrying the young engineer along its length and right to the top of the shaft in Rotherhithe. Water was not the only danger. Foul gases, caused by centuries of sewage lining the Thames, also dogged tunnelling.

As well as physical danger, there was also much anguish on the part of the tunnel's financiers. Money ran out in 1828 when the tunnel was only half-way across the Thames and the government had to bail out the promoters.

The method of boring the tunnel devised by Marc Brunel was the project's one constant success. Having pondered over how ship-worms bore through timber, Brunel devised a tunnelling shield, a giant rectangular structure containing 36 cells each of which housed a miner who delved away at the Thames clay in front of him. When all 36 miners had cut their prescribed volume of clay, the shield was jacked forward and the freshly exposed surfaces lined with a 2ft-thick shell of bricks. These were largely made from the clay dug out by the miners.

The Duke of Wellington, when prime minister, said foreign nations regarded the tunnel as "the greatest work of art ever contemplated". But the Times christened it the "Great Bore" because it took so long to build. Yet when the "Great Bore" was opened, a million people came to walk through it in its first 18 weeks. In 1851, it received more visitors than the Great Exhibition.

Tunnel fever abated within a few years, however, as the ramps needed to bring toll-paying horse traffic through the tunnel were not built. Instead, pedestrians paid a penny a head to walk through an ever increasing number of beggars, buskers and stall-holders. Salvation for investors came in 1869 when the East London Railway Company bought the tunnel to run its trains between Whitechapel and New Cross. This is the purpose the tunnel has served ever since. What took 18 years of back-breaking labour to build is now traversed by Underground trains in just 25 seconds.

In those 25 seconds, passengers see nothing of the artistry of the Brunels' tunnel. They see neither the horse-shoe arches, nor the Greek columns and Egyptian details now covered in soot and brake-dust. Such artistry, however, means nothing to LU. Over the next seven months, it proposes to bang metal lathes into the brick and tile cladding of the tunnel and cover all with concrete. This act of sacrilege is possible because LU has argued that only the shafts at either end of the tunnel are protected by statute and not the tunnel linings. Neither English Heritage nor the Department of National Heritage has objected, which is odd as LU's behaviour suggests that the subsidiary elements of an Ancient Monument are more important than the monument itself.

Is the new work really necessary? The New Civil Engineer magazine inspected the tunnel last February and reported: "The tunnel seems perfectly dry apart from three isolated leaks. There is no sign in the tunnel of the floods of water which the work is supposed to mitigate, none of the bulging, cracking and spalling of an old masonry lining in distress." Will the Channel tunnel, the magazine asks, be in such a healthy working state in 152 years time if it is not maintained?

LU accepts that the tunnel is not dangerous, but argues that works are needed to secure its long-term future. It is not any pressing need, but merely the coincidental seven months closure of the East London Line to allow construction of the Jubilee Line extension through Docklands that has prompted work on the tunnel. Ben Harding of LU explains: "To keep convenience to customers to a minimum, we have decided to do the tunnel at the same time to avoid further shutdowns."

Numerous bodies are now arguing that LU, in its eagerness to utilise the line's imminent seven-month closure, has opted for the quickest and easiest method of waterproofing the tunnel. More careful consideration of the tunnel's long-term future by engineers with more of the Brunels' inventiveness could, however, lead to a solution that would not obliterate one of the greatest engineering achievements of its age. The Newcomen Society, based at the Science Museum, has asked for the tunnel to be spot-listed as a building of architectural and historic interest "so that its forthcoming treatment may be open to scrutiny and proper debate".

This request to the department of National Heritage has been supported by the Georgian Group. James Sutherland, the only engineer member of the Royal Fine Arts Commission, says: "Unless it is absolutely necessary, it should be left as it is." The distinguished engineer Sir Alan Muir Wood, an early critic of LU's proposals and now one of its consultants on the project, concedes that a less destructive option could be conceived if the timescale was more relaxed."As an engineer with a historical sense, I would have liked to have seen another solution," he admits. Alternatives include external grouting, a technique that would have sealed the tunnel from the outside without disturbing its architectural interior. Money might also be spent on cleaning and lighting the tunnel so that Underground passengers could appreciate its fine architectural features.

If LU's works begin next week, then one of the marvels of the 19th century will be lost for ever without even a proper debate. The works will be irreversible, and as Sir Alan ruefully admits: "This is the last we will see of the tunnel." Last week, Stephen Dorrell, Secretary of State for National Heritage, said that the public and relevant experts should be consulted before a decision is taken on whether to list a structure such as the Thames Tunnel. They need to be consulted very quickly, otherwise the legacy of the Brunels will soon be lost from sight. But then, what the eye doesn't see...