Authorities in Germany have warned people to exercise caution around the river Elbe after exceptionally low waters exposed grenades and ammunition left over from the Second World War.

The current heatwave gripping Europe has reduced the Elbe, one of Germany’s three great rivers, to its lowest level in decades, and walkers exploring its banks have stumbled on ordnance that has lain undisturbed since 1945.

There were finds at five separate locations over the last week, and in total 21 pieces of ordnance have been discovered so far this year in the state of Saxony-Anhalt alone, including tank shells, hand grenades and rifle bullets.

“Even after decades under water, ammunition can still be dangerous,” Grit Merker, a spokesman for the Saxony-Anhalt police said.

Sediment can form on the outside of unexploded shells and grenades, hiding the danger beneath, she warned.