At our place we have a big aggregate nest of miner bees that emerges at this time of year. They've been here since before we bought the house (The realtor asked us if we wanted them sprayed before we moved in!
) I believe the species is Anthophora abrupta, the Abrupt Digger Bee. (Not sure why they are called that.) They are the greatest! They are super docile; you could probably trample their nests and not get stung. They build these little chimney entrances to their underground tunnels using mud. They like to gather water for their building on our sidewalk, so we soak it down for them when the puddles dry up.