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But experts said that the preferred theory was that a collapse of the mine's roof in the "goaf" - the void left behind after coal is removed - released methane into the mine.

Goaf, gōf, n. a rick: the coal-waste left in old workings.

You're on the edge of a goaf now.

About her on either side were wastes of black, and in the goaf, by way of clearing, but one thing was discernible, the fealty of Adrian.

Now she would be transported to new horizons where multicolored suns battened on intervales of unsuspected charm, now she would be tossed into the opacity of an abyss where there would not be so much as a goaf for resting-place.