I'm not sure what is digging under the fence, but this is three days in a row. The first day I noticed the hole it was beside one of the fence post and we had a heavy rain - since we had just built the pen I figured the rain had settled the dirt so I filled in the hole. All of the chicks were okay so that was my assumption. The next day I notice a hole in the same spot but again we had rain and the chicks were okay, but I was beginning to wonder. I was telling my husband about it and he wanted to know if I had packed the dirt down tight when I refilled. For the first evening we had no rain and I go out and there is the hole again in the same spot. It's big enough for a cat or possum or rat - but all the chicks are fine. This morning I am going to lay down wire on the inside and stake it down with tent stakes. It's the only fenced area that is outside of where our dog can reach. All other three sides are on the interior of the yard - this is on the privacy fence to the back - we left a wooded barrier to the back of our fence just for sound and privacy - it belongs to us - not sure what might be coming in that isn't bothering the chicks.