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A bomb disposal team is on its way to Leysdown.
Experts have been called to deal with Second World War explosives that have been found.
Police have warned people living nearby to expect to hear a controlled explosion in the next few hours.
A police spokesman said: "Officers were called at 7.55am following a discovery of what is believed to be an unexploded Second World War device at private farm land in Leysdown.
"Police are currently at the scene and a Ministry of Defence explosive ordnance disposal team will be carrying out a controlled explosion later today."
The force added there was no risk to the public.