taru-adjective, adverb taking the 'to' particle
deep and dark; profoundly dark
A literary, somewhat archaic word describing a deep, quiet darkness, often with a sense of mystery or the supernatural. Used as a 'taru' adjective or adverb with と.
森の奥は幽々とした闇に包まれていた。
The depths of the forest were wrapped in a deep, dark gloom.
幽々と続く地下道を歩いた。
I walked through the underground passage that stretched on darkly and endlessly.
Common word for 'deep'; 幽々 adds a literary, dark, and mysterious nuance.
General word for 'dark'; 幽々 implies a profound, almost spiritual darkness.
Reduplication of 幽 (ゆう), meaning 'deep, dark, secluded', forming a literary adjective/adverb. The exact historical derivation is uncertain.