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Ariake Sea is one of the largest tidal flats in Japan.
Literal
Ariake-Sea [topic-は] Japan [possessive-の] most wide tidal-flat [possessive-の] one [copula-です].
有明海 is a real body of water in Kyushu, famous for its extreme tidal range and mudflats. 干潟 (ひがた, 'tidal flat') is specific geographic vocabulary. もっとも広い is the superlative ('most wide'). ~の一つです ('is one of') is the standard pattern for 'one of the [superlative].' A factual geography sentence.