RUBBLE has been found in several cells at H Block 7 in the Maze prison, following the discovery of a sophisticated tunnel burrowed 2.1 metres (seven feet) below ground and extending about 13m (40 feet).
It is thought the prisoners would have had to dispose of 13 tonnes of soil from the tunnel.
A prison officer on routine patrol found a hole in the ground near the perimeter fence after noticing some subsidence, at about 9.30 p.m. on Sunday.
He alerted the authorities and a mechanical digger was brought in which uncovered the underground passageway.
The beginning of the tunnel was traced to a cell holding IRA prisoners. Thirty eight IRA prisoners escaped from the block in 1983.
It is understood the tunnel, which measured 75cm (2 1/2 feet) square, runs just past the perimeter fence of the block to within 30m (100 feet) from the outer boundary wall. Would be escapers would still have had to surmount another fence and wall.
The tunnel is said to have been elaborately constructed and concealed. According to prison sources, it had electric lighting fed from a cell in H Block 7.
It is thought that to shore up the construction the inmates used chair legs and bed boards, which are issued by the hospital to prisoners who complain of bad backs.
About 95 prisoners from Block 7, in a republican wing, were moved yesterday morning to another part of the prison. Visits to republican prisoners at the Maze yesterday were cancelled.
The Maze holds 537 inmates, almost all of them convicted of terrorist offences. They are segregated into loyalist and republican blocks. There are about 300 republicans.
Mr Finlay Spratt, chairman of the Northern Ireland Prison Officers Association, said: "It would appear to be a successfully constructed tunnel. It would appear to me that they could request the bed boards when they wanted them and no one seemed to keep any check on it."
It has also been alleged that reports of noises from underground were not acted upon by the authorities.