Re: Don't know if they exist yet . . .
The subject line was intentional, because the "tubes" could very well be solid.
Yes, the tubes DO exist on the Moon, but there is nothing showing the tubes in question to be actual "tubes", that is to say "hollow." It is likely they are, but that's merely an Earth-bound guess. No one I know of has ever tested a lunar lava tube, but, if feasible, I see no problem with using them.
Cheers,
Dan