A 100 kg (220 lbs) white phosphorus bomb detonated on Friday, causing a huge forest fire in Wittstock.
The blaze consumed some 56 hectares (120 km²) of forest in a former Soviet military training area.
Local residents and workers at a nearby construction site reported hearing a loud blast on Thursday before a plume of smoke started rising over the abandoned forest area.
Firefighters initially suspected a decommissioned power plant as the possible site of the explosion, but failed to find any signs of damage or flames during inspections.
The mysterious bang turned out to be an explosion of an old incendiary bomb at the nearby military range.
The nearly 120-square kilometer site was in use between 1952 and 1993, when much of eastern Germany was part of communist East Germany. The range was used by Soviet troops stationed there for testing weapons.
The bomb disposal on Thursday did not go as planned, as white phosphorous contained in the explosive device spilled out uncontrollably and caused a wildfire.
German firefighters were unable to properly deal with the fire because of the risk posed by the old bombs and shells. Instead they contained the fire and prevented it from spreading outwards.