Okay, so while bumbling on the moon we encounter civilization, in the form of a giant skylight that opens revealing a long tunnel going deep into the moon, and some air is blowing out of it, which seems like a waste of air.
The two boys decide to go down because one of them dropped his helmet down the tunnel (which has air) it but they don't tell the girl first so she probably doesn't know where they are.
They find a shiny rock prism and some bubbling slime soup.