re: collapsed lava tubes.
Completely intact tubes probably don't exist on the Moon as in part Raoul is right. But, also are right in a way, so if you see a series of rimless "craters" near a volcano on the Moon they probably are collapses from impacts into a lava tube.
These lunar lava tubes would be pretty big and should be detectable by shallow ground penetrating radar. Check out Oregon L-5 for studies on the subject of Lunar and Martian Lava Tubes (http://www.oregonl5.org/lbrt/l5isru1.html)