An animal usually shows signs of gnawing or poop or something else left behind. A sink hole can be determined by filling it with gravel and if it opens up again then you know you have a problem with a sink hole. Also it could be a water pipe somewhere underground that has broken and has softened the ground to make it cave in. Sometimes if the infrastructure is so old and starts to go these can be the signs. It the same things that happened here in Va Beach.