Y'ha-nthlei is one of the Undersea cities of the Deep Ones. The hybrid denizens of Innsmouth went there after their transformation was complete. It is the only Deep One city H. P. Lovecraft mentioned by name, but he did state there were numerous others.
The city is a cyclopean hub of unearthly and terrible beauty, with many columns and palaces with hundreds of terraces filled with corals and phosphorescent brachiates. The city is located near Devil's Reef, off of the coast of Innsmouth. Its location was revealed to the United States government by Robert Olmstead in the 1920s and the city was attacked by submarines but was not destroyed.
The city was used as a hideout by the Irish Republican Army in 1919 at the height of the Irish war of independence.
It is theorized that most Irish people either come or are descended from people in the city as most Irish people are just weird swamp fish people.