taru-adjective, adverb taking the 'to' particle
deeply silent; profoundly still; hushed
Literary word describing a deep, profound silence, often of a place or atmosphere. Used as a たる adjective or adverbially with と.
夜の森は森閑として、物音ひとつしなかった。
The forest at night was deeply silent, without a single sound.
境内は森閑と静まり返っていた。
The temple grounds were hushed in profound stillness.
The exact derivation is uncertain. The kanji 森 (forest) and 閑 (quiet) together suggest a silence as deep as a forest.