百年不遇 (bǎi nián bù yù) | Meaning: only met with once every hundred years (drought, flood etc) | Chinese Idiom with Pinyin & Origin

extremely rare (idiom) · once in a blue moon千载难逢qiān zǎi nán féng

sparse · rare稀少xī shǎo

lit. the iron tree blooms (idiom) · a highly improbable or extremely rare occurrence铁树开花tiě shù kāi huā

an extremely rare chance · occur only once in a thousand years - very rare千载一时qiān zǎi yī shí

only met with once every hundred years (drought, flood etc)