It's an entrance to an underground nest of Ground Bees. There are several species around this area and they pollinate your plants! With the several maladies encountered by the European Honey Bee, they, along with the Bumblebee(which, like the Honey Bee, also makes honey), have become important garden allies. If you can, leave the entrance undisturbed, so that they can come and go. They are not agressive, so there is little danger of being stung.
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