How do I remove an underground paper hornets nest?
Yellow Jacket Hornet nests are seasonal in nature so you can try one of several methods.


Wait it out till fall or winter and dig the nest out once the colony has died. Fix the terrain so there is no ground void that can be used to reestablish a nest next year.
Try a bit faster approach, wait till after dark and dust the nest with a powder of boric acid and pyrethrin which will take time to be disbursed in the nest by the hornets and may take a couple of applications. The combination should be bio-safe as the boric acid will bind in the soil before getting down to the water table. Avoid getting stung, don't shine any lights directly into the opening. A wetter, messier solution is mint oil mixed in a hot, soapy water solution sprayed over as much of the nest as you can get at, but damp solutions don't always work as well.
Fastest, have a professional use a metered amount of Sevin, Deltamethrin or similar dust insecticide accurately dispensed into the nest. Have them use the lest likely to contaminate ground water.




University of Idaho document with underground nest cross-section