People often ask me how to find ground-nesting bee nests. In reality I see nests in the ground all the time but most are very tiny and aggregations are fairly infrequently encountered. Here is a time-series of a Lasioglossum sp. female at the entrance of her nest, likely only 2-3 mm in diameter. She quickly saw off the more brightly colored male.
Here's a few thoughts, observations, and just cool entomological things to share. Enjoy!