From the back cover of the book:

"Far below the darkest of the dungeons, in those regions where the echoes of mankind's feet are seldom heard, where even firelight is a bizzarre intruder from a long-forgotten place, the foes of humanity breed and multiply...

The Cyclopean Deeps is a high-level campaign "sandbox" in the Under Realms near Rappan Athuk. Here you will find the dark city of Izamne, the Domes of the Serpentfolk, and even the twisted double-reality where the material plane touches upon the bizarre Plateau of Leng. Dark secrets abound!"