BBHJ, bumbles do nest in the ground, but in my experience, they do so in an area that is not frequented regularly by human or I'd imagine other animal activity. It doesn't take long for them to find an area that has been abandoned though. Timing seems to be everything. After baseball season when I was in my teens, ocassionally we would get them in the in-field the following season. They would set up their colonies after the season was done, and when the next season started we would have them every where. I'm not sure how the playground people dealt with them, I guess they would just spray the field.