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.
This research is still uncharted territory.
Living abroad was unfamiliar terrain for him.
未開の地 emphasizes a place that is undeveloped or uncivilized, while 未知の領域 focuses on being unknown or unexplored, and can be used figuratively.
未踏の地 specifically means land that no one has set foot on, whereas 未知の領域 is broader and includes abstract domains.
Compound of 未知 (unknown) + の (possessive particle) + 領域 (area, domain). The phrase is transparent and follows standard Japanese grammar.