Underground pests......  Spray some foam insect killer around the hole at night, to prevent the sentinel hornet from waking up the herd.  Then pour a little gas down the hole, cover the hole with a rock.  The gas fumes will kill them, the foam will kill the ones that almost escape.
Aim a flashlight in the general area of the hornet nest, leave the flashlight there.  Come in from the other side.  itll distract the sentinel hornet.  Suckers love my wife's pool.