Moondust

I have seen people online asking "Why didn't they go back to the moon?" and proposing all sorts of way-out theories.  Well, none of those are necesary - the simplest answer is very practical and very deadly: Moondust.  Have a look at this :

No angry aliens, mysterious glass structures or secret bases required.  

Unless you go to some serious effort ( e.g. building an elevated landing pad and sealing your base very, very thoroughly) you are bound to end up with leaking spacesuits, failing equipment and dead astronauts - it is only a matter of time. 
No wonder people on Earth now look to Mars as a much better colonisation option: at least it has atmosphere and weather so there will be erosion and you won't have that sandpaper dust problem (although Mars has it's own problems).