Ny'alotha

Ny'alotha the sleeping city is a place that is mentioned in whispers from Yogg-Saron to players who attempt to solve his puzzle box.

Whispers

 * Whisper
 * Whisper
 * Whisper

Emerald Dream
Based on the whispers' explicit descriptions of the city Whisper it is possible that Ny'alotha is located within the Emerald Nightmare, but may have originated from somewhere on the world of Azeroth.

Forest
Ny'alotha may be hidden in a forest, based on the mysterious whisper Whisper A few more whispers mention dark woods, including one that may indicate the presence of an old god: Whisper This could also mean Karazhan.

Beneath the sea
It may also be one of the sunken cities similar to Vashj'ir and under control of Naga and Queen Azshara, hence the : city of old, terrible, unnumbered crimes.... Many things that rest under the ocean's surface are called "sleeping". This speculation is supported by the whisper Whisper

Another puzzle box whisper states Whisper, suggesting that if Ny'alotha is indeed sunken, a creature there lies dormant. It may even be an old god: Whisper

Inspiration
The name Ny'alotha may be derived from of H.P. Lovecraft's Cthulhu Mythos. Notably, one of the quotes ("city of old, terrible, unnumbered crimes") is similar to a line from the short story Nyarlathotep. It may also be a reference to R'lyeh, the underwater city where Cthulhu slept, dreaming.