adjective
an adjective describes or modifies a noun or pronoun, in this case 'undiscoverable' describes something that cannot be discovered
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The undiscoverable truth about the ancient civilization remains a mystery to this day.
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The undiscoverable treasure is rumored to be hidden somewhere in the old abandoned mansion.
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I heard there's some undiscoverable stuff at the bottom of the lake, wanna go check it out?
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Her true feelings were like an undiscoverable treasure buried deep within her heart.