What kind of bee digs in ground?

I got stung twice this morning while weeding among the branches of a low-growing yew in front of my house. Sunsequent observation indicated an undeground nest of bees, which made me ask the same question. Here's a pretty decent link:

http://ohioline.osu.edu/hyg-fact/2000/2143.html

There are many other sites with similar information, photos, etc.