What cause pasture holes to develop? Holes will appear when my horses walk around contained by their pasture,...

Holes will appear when my horses walk around contained by their pasture, some of these holes are very thoughtful. No sign of animal inhabitants like rabbits or woodchucks at adjectives. I've been riding when the ground colapses below my horses hoof into this deep hole. No unfolding where they will show up! What is cause them and how do I get rid of them. I'm afraid to turn my horses out for mistrust they will break a leg!
What are Thermopane Windows?
Answers:    Moles, rabbits, ground hogs, any burrowing type rodent or animals. Some have a small make friends of tunnels with several entrance points, so when you're trying to find at them at one entrance they run to another entrance poke their heads up and keep under surveillance what you're doing. A good sign that the soil in that is rich and attracts their food source, whether it be worms grubs or vegetation.
Hope this be helpful, ;-)
Garden Question: Is this a pole bean or...
Burrowing animals could do it, or other animals digging holes, such as dogs, coyotes or the similar to. I'm sure your horses will be fine. Wild horses deal near it. Have your land checked, I guess. We own them, and we have cows that dont really come across to get into much trouble.