*****Sorry for the long delay - I just forwarded the question to the designer and the official answer is yes when you play a path card it must be able to trace a path back to the start card.
Which means in a cave-in situation you must fill in the hole before continuing that path.
Thanks,
Zev Shlasinger, President
Z-Man Games, Inc.
www.zmangames.com
http://www.boardgamegeek.com/article/836119#836119
