This may seem a strange question, but I can't seem to find the answer anywhere else. If a square of blocking terrain generated by the Barrier power is destroyed it leaves behind a debris marker. My question is, does the debris disappear with the rest of the Barrier, or does it remain?

Player's Guide, page 7, under Barrier:

Blocking terrain markers can be destroyed as per the rules for destroying blocking terrain. They would be replaced with rubble tokens when destroyed. The rubble tokens are removed from the game the same time that the normal blocking terrain markers are removed from the game.

Well there you go, nice and official like. Thanks for the quick answer.