How do we kill underground wasps?

Sevin powder is the right choice..... you already know where the entrance hole is, so at dusk, when they are mostly all in for the night, pour the Sevin into a 'cereal' bowl sized disposable bowl, like a butter bowl, maybe.... this is so that you can pour a lot, quick!!.....then go to the hole softly and quietly and pour the Sevin all around the hole, so that the bugs MUST walk thru it to get in or out of the nest.... get it around the crack and on the ground, too....then wait..... in 24 hours, if the Sevin is all around the entrance, the nest should be dead..... at that point, turn over the dirt to bury the Sevin.... wash it off with the hose from the patio and cement, turn that over, too... add some new dirt over what you've turned.... there might be a few stragglers come to look for home, but they'll go away when they can'f find it......

Rather than using spraycan wasp killer, I believe you need to use wasp powder - google for it and you'll see the product I mean. Here in the UK it's available cheaply, like USD$4 for a quite large plastic bottle; it usually comes with a hole in the top to pour/squirt the powder; you lay it at the entrance, pouring/squirting it liberally into the hole(s) where the wasps enter and exit - so they pick up the powder as they go in and out, and even a tiny grain will kill an individual wasp. It will be all over in 24hrs.

find some silica powder,it takes the moisture out of the wasps and nest killing them,if you cant locate any from a pet store or hardware store.I would suggest calling an exterminator for underground wasps,they are a mean critter for sure and the nest may be further in than the sparay can reach

Pour about a quart of Clorox into the crack, slowly so it will all seep in. Now cover the crack with dirt. Do this after dusk, when they are quiet.

***You're right, flooding doesn't usually work well. The Clorox will infiltrate the air pockets and kill those babies, as long as you cover the crack with dirt.

I would say seal the crack almost all the way on the fountian then flood it with water long enough to drown them all.

i am no expert but could you possibly seal up the crack?

fill the hole with water and plug it.