An 18th-century account of the city claimed that medieval Manchester had been protected by a massive defensive ditch, but modern academics had considered this highly unlikely. They maintained that pre-industrial Manchester was simply too small to need such defences.
Now, however, a team from the University of Manchester archaeology unit has discovered just such a ditch, 30ft across and 15-20ft deep.
Together with the rivers Irwell and Irk the 3,000ft ditch would have defended a substantial area of some 40 acres. Adjacent to it there was almost certainly a stone wall or palisaded bank.
The ditch was discovered during current redevelopment work necessitated by the IRA bomb in June last year.
Historians know that between the 1st and 4th centuries AD Manchester (the Castlefield area) flourished as a small Roman fort and town called Mamucium, a contraction of the words "breast hill", named after the hill on which the town stood.
Then, in Anglo-Saxon times, Manchester was re-established about one and a half miles to the north of the old site. The bottom part of the ditch may date from late in the Anglo-Saxon period when, in 923AD, chronicles records that King Edward the Elder sent troops to Manchester "to repair and garrison it".
However, the top part of the newly discovered ditch appears to date from some time in the 13th century and archaeologists found that it was filled with leather off-cuts - waste material from what seems to have been Manchester's until now unknown first industrial revolution.