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cavity; hollow; void
hollow (voice, smile, etc.); blank (eyes, look, etc.); vacant (expression, stare, etc.); empty (words, heart, etc.)
長い戦争で街は洞ろな廃墟と化していた。
The long war had turned the city into a hollow ruin.
彼は洞ろな目をして何も語ろうとしなかった。
He stared with vacant eyes and would not say a word.
大木が枯れて幹の中が洞ろになっていた。
The great tree had died and its trunk had become hollow.