A resident discovered the explosive Thursday afternoon while fetching newspapers from a mailbox in Sundby village. The nearest house is just ten metres away from the mailbox.
Police are investigating the matter as a case of endangerment and unlawful threat. So far no suspects have been identified.
According to Detective Inspector Ove Storvall, the grenade could have caused extensive damage or even death if it detonated.
Police speculate that the device is actually light mortar ammunition.
An army representative was due to retrieve the grenade Friday. Sundby lies about 10 kilometres south of Pietarsaari.