Who's Digging in my Pot?

I went to water the flowers on the deck and found this.

Disconcerting, to say the least. I don't have any more potting soil to fill in this hole. I rearranged what was there and watered deeply, but it's really too early for squirrels to be burying food for winter. I've had this geranium for years and would hate to lose it to some pot digging critter.

Beverage:  Water

Deb