井の中の蛙大海を知らず[inonakanokawazutaikaiwoshirazu] parochial
井桁[igeta] well lining
井戸[ido] water well
井戸屋形[idoyakata] well roof
井戸掘り[idohori] well digging

井戸綱[idozuna] well rope
井戸車[idoguruma] well pulley
井戸水[idomizu] well water
井戸側[idogawa] well curb
井戸替え[idogae] well cleaning

御守[omamori] charm
井守[imori] newt (esp. the Japanese fire belly newt, Cynops pyrrhogaster)
看守[kanshu] jailer
看守者[kanshusha] jailer
居留守[irusu] pretending to be out

見守る[mimamoru] to watch over
厳守[genshu] strict observance
固守[koshu] clinging
攻守[koushu] offense and defense
子守り[komori] babysitter

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