"百年不遇" 英语有一单词,怎么说来着??

rarely occur once in every 100 years.
EXP:This summer, while our country was forging ahead with plans and undertakings in fulfilment of the wishes of the people, it faced the test of the severest floods in a century.
今年夏天，正当国家事业按照人民的意愿，大步向前迈进的时候，国内发生了百年不遇的特大洪水灾害。

Under the circumstances of aggravating Asian financial crisis and the huge floods that rarely occur once in every 100 years, it is not easy for China's economy to make these achievements and maintain such a good momentum of development.
在亚洲金融危机不断加剧、遭受百年不遇的特大洪涝灾害的情况下、中国经济取得这样的成就，保持良好的发展态势，来之不易。

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once in a blue moon

...of the centuary

once in a blue moon