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

火口湖[kakouko] crater lake
湖沼[koshou] lakes and marshes
湖上[kojou] on the lake
湖水[kosui] lake
湖畔[kohan] lake shore

湖面[komen] lake surface
江湖[kouko] the public
人造湖[jinzouko] artificial lake
中禅寺湖[chuuzenjiko] Lake Chuzenji

御爺さん[ojiisan] grandfather
老爺[shinpu] one's father
狸爺[tanukijijii] cunning old man
好々爺[koukouya] good-natured old man
爺が背[hizaragai] chiton (any marine mollusk of the class Polyplacophora)

エロ爺[erojijii] dirty old man
糞爺[kusojijii] old man
爺さん[jiisan] grandfather (may be used after name as honorific)
洞爺湖サミット[touyakosamitto] G8 Hokkaido Toyako Summit