Explosive fox deterrent

Probably the most curious sign I have ever seen in my life. I was walking along a small country lane just outside Guildford in England, when I came across a sawmill. Outside, on the fence were several signs, the most amusing being:

WARNING!
Explosive Fox Deterrents
Do not Enter

Surely they're illegal. They're mines! It's as much to keep people out as the foxes.
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