What is under your swing set? Wood mulch? Rubber mulch? Grass? Rocks? Something else?

Our new house has a great swing set in the yard, but it is a complete mud pit underneath it, to the point that I loathe sending the girls out to play because they come inside such a mess! I like the "idea" of rubber mulch, but am reading conflicting things about whether or not it's really healthy for kids to be coming into contact with it.