Strengthen the immune system. Boosts the immune system by being exposed to millions of microbes.
Boosts happiness. Bristol University's study found that the bacteria mycobacterium vaccae increases the production of serotonin, a contributor to feelings of well-being and happiness.
Detox the body. Clay masks draw impurities from the pores.
The first two benefits alone should be enough to get you rolling around in the mud (or at least not be scared of it!).