Pieces of lava tube ceiling or wall that have broken off and fallen to the floor. Much of this breakage occurs during the cooling phase, as shrinkage of the tube's lining occurs and contraction cracks form. Some ceilings in lava tubes contain very unstable rock that has not yet fallen to the floor, and caution must be taken during exploration not to dislodge it. 
Breakdown of the ceiling may also make an opening to the surface, or skylight.
In the photo below we see breakdown that came from the collapse of the pahoehoe floor of an upper level. Clearly, the breakdown occured after the last sreams of molten lava had coursed through the lower level, or they would have been covered and possibly become welded breakdown.

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Created: August 4, 2000
Author: Dave Bunnell