taru-adjective, adverb taking the 'to' particle
Literary, somewhat old-fashioned word describing something stretching endlessly, such as a plain, desert, or ocean. Often used in descriptive, poetic, or formal contexts.
漫漫たる砂漠が目の前に広がっていた。
A vast desert stretched out before my eyes.
彼は漫漫と続く草原を眺めた。
He gazed at the boundless grassland.
広大 is a common, neutral word for 'vast' or 'extensive', while 漫漫 is literary and emphasizes endlessness.
果てしない is a more everyday expression for 'endless', whereas 漫漫 is formal and often used in written descriptions.
The exact derivation is uncertain; the kanji 漫 (man) carries meanings of 'spreading' or 'overflowing', and the reduplication reinforces the sense of boundlessness.