well i was out today,had 5 bolts then 1 of my jills did not come out,she was down for 1hr10min no signal at all so i put the other jill down with a collar on ,down the same hole 5 min past the 1st jill came out with no collar,so pin pointed were the other jill was dug 3ft got her out and the other collar was in there to,but no kill but i think they killed some were causing a blockage and the 2nd jill moved it abit but she must of got traped,so i would of been at a loss if i did not use a collar today with my ferrets ...