Identifying Burrowing Flying Insects?

In the beginning of April we found small mounds of soil with a small hole in the middle. They were all in a round soil circle that we dug out in the lawn. We have seen a fly that came out and it was the shape of a house fly, but bigger. It has clear wings, and it's body is a rusty brown down each side and the rest, is black.

We have tried putting all sorts of products down the holes, including ant powder, but nothing works. My husband drenched the soil completely, but then the next day found 5 more holes. A few made holes in the lawn next to the soil patch. We're worried as we don't know what they are, and also how many eggs they are laying. Is there anything we can kill them with? And would it be safe to dig up the soil in winter to take any nests and eggs out?

Thank you.