Archaeologists from the Museum of London have discovered the foundations of an 800-year-old water mill in south-east London.
The 12th Century water mill found at Greenwich Wharf is believed to be one of the earliest tide-powered mills to be found in the capital.
The 10m (32.8ft) by 12m (39.3ft) base of the mill supports a 5.2m (17ft) wheel. Both are well preserved.
Diggers chanced upon it while digging for a housing development.
The riverside peat deposits preserved the large piece of intact wheel and an enormous trough made from a single oak beam.
Even the carpenters' construction marks are visible on the centuries-old wood.
The remnants of the tide mill has been dismantled, and key sections have been moved to York Archaeological Trust for conservation work.
Simon Davis, contract manager for Museum of London Archaeology, said: "Tide mills may have been numerous along the Thames foreshore in the early medieval period.
"However, little evidence of mills in use in the early medieval period has been found on archaeological sites, so the discovery of a 12th Century tide mill at Greenwich is very significant and exciting."