While researching some geocaches in the Jackson, MS area, I saw the following note contained in several caches:
PMO = Premium Member Only. I thought this was a neat idea and made me realize that as a Premium Member, I am supporting Geocaching.com! And the website has been pretty good to me over the last few years, giving me the key to hours of enjoyment in the great outdoors almost every weekend! -- ORROriginally a PMO Cache (Just for 30 Days!)
This was initially listed as a Premium Members Only cache as I think all geocachers should do their part to support Geocaching.com. It was opened to all members 30 days after being published.