expression, noun
uncharted territory; unknown waters; unfamiliar terrain
Used both literally for unexplored physical areas and figuratively for new fields, experiences, or situations where one lacks knowledge.
この研究はまだ未知の領域だ。
This research is still uncharted territory.
彼にとって海外での生活は未知の領域だった。
Living abroad was unfamiliar terrain for him.
Compound of 未知 (unknown) + の (possessive particle) + 領域 (area, domain). The phrase is transparent and follows standard Japanese grammar.