A bomb disposal team called to deal with a sea mine uncovered a second explosive as they cleared the first.
They were first alerted on Monday to the Mark 6 sea mine at Thornham Marsh in Norfolk, and returned at low tide on Tuesday to detonate the device.
As they cleared the area around the mine, they found second a high explosive shell. Both were destroyed in controlled explosions.
Hunstanton coastguard reminded walkers on the marshes to be vigilant.
The coastguard was asked to meet Norfolk Police officers at 10:15 BST on Monday, after the device was discovered by members of the public.
They located the device using the police drone and the What3Words app.
A 500m cordon was set up until the tide had covered the area again, and the team returned just before 12:00 on Tuesday to complete a controlled explosion.