FYI - when looking at my personal achievements it says I have 3 hidden left to discover. However, if you look at the global achievements list it shows all available achievements including those that are marked hidden in my personal achievements. Not that it helps much since i need to discover 3 locations and i don't know where they are. :-) On a side note - one of the hidden achievements is to discover a "drill rover" which i thought i had done already.
Customer support service by UserEcho