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.
I am a Postdoctoral Associate at CU-Boulder and have lead a number of projects studying patterns of native bee diversity in Colorado. Please enjoy your visit and contact me if you have any questions.