Workers digging on the underground network in the western city of Düsseldorf have uncovered a 34-kilogramme woolly mammoth tusk over 10,000 years old, city officials said.
Excavation work was stopped immediately, while the 1.20-metre-long tusk was gently removed and taken away for scientific study, Düsseldorf authorities said in a statement.
The tusk was the only part of the animal found during the dig, some 12 metres below the surface.
Woolly mammoths died out in the region around present-day Germany some 10,000 years ago.