Néng qīnyǎn kàndào rìquánshí, zhēn shì bǎiniánbùyù de jīhuì.
Seeing a total solar eclipse with your own eyes is truly a once-in-a-lifetime opportunity.
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usage
Often used for natural disasters (floods, droughts, earthquakes) to stress severity. Also used loosely for rare positive opportunities, though the disaster sense is more common in news.