"...recent studies indicate that treatment with a specific soil bacterium, Mycobacterium vaccae, may be able to alleviate depression." Check out this report.
I don't know how much Prozac costs, but dirt is definitely cheaper! Of course, we need to take the results of these studies with a grain of salt. And mice are not humans. But still! I mean, we all know that gardening is good for the soul. But who ever thought that inhaling soil bacteria would be the source of that "goodness". Very interesting.