When our terriers go to ground, they can meet the unexpected.
Misto's sire was killed by a porcupine that was located underground in New York in an area where they are not commonly found.
Sailor, my late great terrier, was paralyzed by a black widow spider bite -- a bite that I am sure weakened her heart, and led to her death some months later.
And then, there is the possibility of seriously venomous snakes, such as this den of timber rattlesnakes near Bath, New York.
The good news is that in my area the most poisonous snake is the Copperhead, and it's not too toxic.