I'm with D&b on the ordnance issue , esp. old (& unstable) stuff .... this could include unexploded &/or unfired artillery rounds , plain 'ole ammo (tracers too) , fuses , grenades & all the 'non-sweetness and light' items used on battlefields .
I have been in minefields & don't like them .
Believe it or not , I have found many ferrous metal objects via using a good quaility liquid-filled compass , slowly 'scanning' @ near ground level .
Found a few cool items (Italian 1909 pistol , for one) & lots of crap (nails,screws,hunks o' junk , etc.) .