Emergency services are evacuating people from their homes in Derbyshire over fears a reservoir could collapse following heavy rainfall in the area .
Toddbrook Reservoir is on the north-west edge of the Peak District National Park, sitting above the small town of Whaley Bridge.
Some experts say it was built in 1831, while the Environment Agency record it as being built between 1840-41.
The structure supplies water to the Peak Forest Canal, a waterway in northern England running between the town and Ashton-under-Lyne.
Owned by the Canal & River Trust, the reservoir is a Site of Special Scientific Interest (SSSI) due to local wildlife.
The spillway on the embankment dam, which releases water, became damaged following extensive rainfall on Thursday and partially collapsed.
Reservoir safety is maintained by inspections under an act created in 1930 and strengthened in 1975, according to experts, but flooding and other weather events have led to concerns about safety of older structures.
Professor Roderick Smith, from Imperial College London, said: "Extreme weather events mean that there is increasing unease about the safety of older dams: particularly the need to release excess water safely and easily."
The reservoir was damaged due to flooding in 1964, according to the Environment Agency, but another specialist said it was "unlikely" it had been in an unsafe condition before the heavy rainfall on Thursday.
Professor Tim Broyd, Professor of Built Environment Foresight at University College London, said: "Dams are highly regulated structures, which includes regular structural inspections by highly qualified engineers.
"It is unlikely therefore that the dam was in a previously unsafe condition.
"What may have been the cause, however, is that the flow rate into the reservoir was exceptionally high, as a result of extreme local rainflows."