R'lyeh (ルルイエ Ruruie?) is a fictional sunken city in the Pacific which is featured in H. P. Lovecraft's short story "The Call of Cthulhu". Trapped within the city is the entity known as Cthulhu. In Toaru Majutsu no Index, the city can be brought into existence through certain spells based off the Cthulhu Mythos.