[verb (plain)] + と言われている [i-adj] + と言われている [na-adj] + (だ) + と言われている [noun] + (だ) + と言われている
A noun or a な-adjective takes だ before と, or nothing; it never takes な.
After a noun, the pattern gives what someone or something is known as. With nothing between the noun and と, it reads "is called".
Hearsay そうだ passes on something you heard or read – from the news, from a friend. と言われている states a belief that is widely held, so it sits in explanations, guidebooks and the news.
These go beyond the plain present.
In polite speech the ending is と言われています. と言われていた puts the belief in the past – it was said then, and may not be now.
With a person and に, the same form means "I have been told" by that person – no longer people in general.