taru-adjective, adverb taking the 'to' particle
Literary, often used in descriptions of expansive landscapes or abstract concepts. Functions as a taru-adjective or adverb with と.
広漠とした砂漠が目の前に広がっていた。
A vast desert stretched out before my eyes.
彼の知識は広漠としていて、どこまで深いのか分からない。
His knowledge is so boundless that I cannot tell how deep it goes.
Composed of 広 (wide) and 漠 (vague, vast). The exact historical derivation is uncertain, but the combination conveys an expansive, boundless quality.