fountain of youth
US /ˌfaʊn.tɪn əv ˈjuːθ/
UK /ˌfaʊn.tɪn əv ˈjuːθ/

1.
a mythical spring that restores the youth of anyone who drinks or bathes in its waters
:
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Explorers searched for the legendary fountain of youth for centuries.
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Many people dream of finding a real-life fountain of youth.