taru-adjective, adverb taking the 'to' particle
Literary adjective describing something expansive and limitless, often used for oceans, skies, or abstract expanses. Functions as a taru-adjective (e.g., 洋々たる) or adverbially with と (e.g., 洋々と).
洋々たる大海原を前に、彼は言葉を失った。
Before the vast expanse of the ocean, he was at a loss for words.
彼の未来は洋
His future seemed to stretch out boundlessly before him.
茫洋 also means 'vast' or 'boundless' but often carries a nuance of haziness or obscurity, whereas 洋々 emphasizes clarity and openness.
Composed of 洋 (ocean, vast) reduplicated for emphasis. The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of vastness.