In my garden I have noticed bees burrowing into the ground. They're not yellow jackets and don't look like normal wasps or hornets. They seem to be kind of fuzzy and grey/black in color, maybe the size of a honey bee. This is the second year they've been in the same location. They make little one-bee-sized holes that are sort of around two huge holes the diameter of a golf ball, where lots of bees go in and out. They get mad whenever I weed or dig at the dirt, but I haven't been stung yet, so I don't know if they're dangerous. I think their nest is mostly under a large flat stone (but I won't pick it up to look!)

But I need my garden back. Any ideas on what they are and how to get rid of them (preferably without damaging/contaminating the soil too much)? Is it something I can do myself or should I call an exterminator?