Dens in the Bible

Judges 6:2 And the hand of Midian prevailed against Israel: and because of the Midianites the children of Israel made them the dens which are in the mountains, and caves, and strong holds.
Hebrews 11:38 (Of whom the world was not worthy:) they wandered in deserts, and in mountains, and in dens and caves of the earth.
Revelation 6:15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;