This is the 8068th most frequent Polish word.
nieznana
"Nieznana" means "unknown" in English.
'Nieznana' is used as an adjective describing 'przyszłość' (future), indicating that it is not known.
Przyszłość jest dla nas nieznana.
The future is unknown to us.
'Nieznaną' functions as an adjective in the accusative case, describing 'jaskinię' (cave) which is the direct object of the verb 'znaleźliśmy' (we found).
Znaleźliśmy nieznaną jaskinię w górach.
We found an unknown cave in the mountains.
'Nieznana' is an attributive adjective describing 'piosenka' (song), used in the nominative case as the subject of the sentence.