An almost-deserted fishing town in Massachusetts. It was once dominated by degenerate Cthulhu cultists, led by the evil Obed Marsh and his Esoteric Order of Dagon church. For decades, they held the town for ransom, murdering their foes and perverting their servants with blasphemous religious rites and unholy unions with the amphibious deep ones.
Since the government raided the place and torpedoed the reef offshore back in 1928, it's been nothing more than a decaying, rotting, legend-haunted ghost town, stinking of fish and sin.
"The Shadow Over Innsmouth" by H.P. LovecraftEncyclopedia Cthulhiana by Daniel Harms, pp. 102-103