"French Battlefields especially dangerours after 100 years"
Explosives get more dangerous with age. Cordite come to my mind. With regard to the shells, the big one is from Dora.
Yes, and no. Aged (ie cracked and/or exeduded) explosives tend to be more sensitive such that major mechanical assaults would set them off, but other than a few notable cases they don't become exceptionally sensitive, so the primary risk always remains the fuse.
It's been well accepted that particularely hostile environments like jungle/tropics actually effectively inert many explosive devices like mines by just utterly destroying the fusing system via corrosion and infiltration of debris and fungal growth.