Judges 6:2

And the hand of Midian prevailed against Israel, rested heavily upon the people; and because of the Midianites the children of Israel made them the dens which are in the mountains and caves and strongholds. They made use of the natural grottoes and caves in the limestone, excavated others, made them habitable by digging air-holes from above, and fortified many, to serve not only for retreats in case of a raid, but also as places for the safe-keeping of their personal property.