Mine

The is an invisible hazardous object in  which can only be found in the Park in grassy areas. It spawns more often near to military outposts.

It can't be damaged by melee or projectile weapons (though can be set off with rocks), and, presumably, can be blown up.

When a character or item gets close enough, it will commence a visible 3-second countdown (with audio 'beeps' as well) before it explodes.