The outline of a fort built during the English Civil War has become visible after heavy rains caused flooding to fill a moat around the earthworks.
The Earith Bulwark was built in about 1643 at Earith in Cambridgeshire by Oliver Cromwell's forces to protect crossing points on the local rivers.
Historic England said it was "amongst the most elaborate fortifications" from the Civil War (1642-51).
Many parts of the county have had flooding and more rain was forecast.
The central section of the Earith Bulwark covers about 60 square metres and is situated to the east of Earith.
It was built on a narrow strip of land separating the man-made Old and New Bedford Rivers, both of which are fed by the River Great Ouse, which is about 150m to the south.
According to Historic England's website the fortification demonstrated "the importance of the Earith crossing during the Civil War as part of the military frontier surrounding the Isle of Ely".
The fort was also the site of a defensive gun emplacement during World War Two (1939-45).
While some Cambridgeshire residents have been dealing with floods caused by heavy rain last week, Storm Christophe could bring more disruption to the region.
The Environment Agency currently had 11 flood warnings - meaning "immediate action" is required - in place across England, stretching from the south east to the north east.
Cambridgeshire Police warned people not to drive through floodwater and the county's fire service warned of the dangers lurking beneath areas covered with standing water.
Even just a small amount of water is enough to take you off your feet. It could also be contaminated with sewage water.— Cambridgeshire Fire and Rescue Service (@cambsfrs) January 18, 2021
It’s not worth the risk! Keep out of flood water ⛔️