It was only when the EOD engineers inspected the mine did they encounter the second item of unexploded ordnance. This turned out to be a small wartime artillery shell. The driving band at the base of the shell has grooves imbedded, confirming that it has been fired and fell back to earth unexploded.
There was a seaward coastal firing range to the east of Thornham historically. Furthernore, a number of anti-aircraft (AA) batteries occupied the wider area during WW2. It is likely therefore, that this shell was fired out to sea from a coastal gun and over time, marine currents deposited the projectile on the marshland. Although the photograph does not give a sense of scale, it resembles a WW2 40mm AA shell, used by many LAA regiments for home defence.
The same can be said for the naval mine which would have been originally laid in an offshore minefield many kilometres away. Thousands of Mark VI mines were sewn in British waters during WW1 as a defence measure against German U-Boats (submarines). Hundreds of Allied merchant ships were sunk by U-Boats in the North Sea between 1914 and 1918.
British Mark VI Sea Mine
The Mk VI was an 86cm diameter spherical contact mine. It contained a buoyancy chamber, firing mechanism and a 300lb (140kg) high explosive charge. The Toxyl main charge was cast into the lower hemisphere. Toxyl was a mixture of 60% TNX and 40% TNT. It was tethered to an anchor which allowed the mine to float up to a predetermined depth.
Post-war mine clearance involved severing the tether cable, resulting in the mine floating up to the surface where it was destroyed by rifle fire. However, many mines did not explode, instead sinking to the seabed.
These mines were not the biggest to be deployed in British waters historically, however their 140kg explosive charge does of course pose a very serious hazard.