It's often noted that getting out into nature is good for the soul, both from physical and mental health standpoints. A team of researchers in Australia has put a dollar figure on the mental health benefits, saying that protected areas such as national parks around the world provide $6 trillion in benefits.
"Nature exposure improves human mental health and wellbeing. Poor mental health imposes major costs on human economies. Therefore, parks have an additional economic value through the mental health of visitors," they write in the abstract to their paper, Economic Value of Protected Areas Via Visitor Mental Health.