An extensive network of medieval canals which were used by monks in punts have been discovered in the Lincolnshire fens, researchers revealed.
Around 56 miles of waterways, which are now blocked by silt and hidden in the fen landscape, were found using aerial photographs, the Royal Geographical Society's annual conference was told.
It is thought the canals, which would have been 20ft to 40ft wide, were built by the monasteries in the area after 9th century raids by Vikings who destroyed many monastic sites.
Civil engineer and archaeologist Martin Redding said the schemes were unlikely to have been created for drainage alone because of the huge costs involved.
Instead they would have been used first to ferry locally-quarried stone to rebuild the monastic sites, which belonged to orders including the Benedictines and Cistercians.
They would then have been used to carry the rich resources of the fens to market in "fen lighters", which are shallow, flat-bottomed boats.
The cargo could have included cranberries, as research on a now extinct acidic peat bog in the Lincolnshire Fens has confirmed it would have been an ideal area for growing the fruit.
Mr Redding, a member of the Witham Valley Archaeology Research Committee, said it is likely each monastery had its own network of canals connecting parts of its estate including its farms.
Mr Redding said the canals showed "breathtaking engineering projects" were being undertaken in the fens 800 to 1,000 years ago.
He added the canals would have lasted until around the 14th century when rising sea levels would have made their operation increasingly difficult, while the dissolution of the monasteries in the 16th century probably finally ended the system.