空洞[kuudou] cave
空洞化[kuudouka] making hollow
鐘乳洞[shounyuudou] limestone cave (cavern, grotto)
洞窟[doukutsu] cave
洞穴[douketsu] cave

洞察[dousatsu] discernment
風洞[fuudou] wind tunnel
空洞化現象[kuudoukagenshou] (economic) hollowing-out phenomenon
洞見[douken] insight