We had a sink hole open in our pasture after 5 inches of rain in our area years ago. One of our cows fell into the hole. She had broken her neck so her life ended quickly. My husband almost ran out utility vehicle into the hole, there was brush around the hole so it was hard to see until you were right onto it. With the sink holes around here we cover them back up & divert the water flow into another direction. Under the clay ground around here there are limestone formations with cracks in them & caves under the formations. When it rains sometimes the cracks open up & the clay ground caves in forming a sink hole.