This is the 13635th most frequent Russian word.
неизвестного
"Unknown."
In this sentence, 'неизвестного' (the unknown) is used as a noun derived from 'неизвестный', emphasizing something abstract or unfamiliar.
Он оказался перед лицом неизвестного.
He found himself facing the unknown.
Here, 'неизвестного' serves as an adjective modifying 'острова' (island), denoting that the island is not known to the speaker or group described.
Мы встретили человека с неизвестного нам острова.
We met a person from an island unknown to us.
In this example, 'неизвестного' functions as an adjective describing 'источника' (source), characterizing it as unspecified or undefined.