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Baseball is said to have started in Cooperstown in 1839.
Literal
Baseball [topic-は] 1839 [at-に] Cooperstown [at-で] be-started-[passive-られた][quotative-と] is-said-[passive generic-いわれている].
~といわれている ('it is said that X') is the passive of 言う with ~ている, forming an impersonal 'it is said.' Common for reporting common beliefs, folklore, and historical claims. クーパーズタウン is Cooperstown, NY — the mythic birthplace of baseball according to the (historically disputed) Abner Doubleday legend.