In 2087, the earth was swallowed by the "Grey Domain" - a strange substance formed by the symbiosis of out-of-control nanomachines and quantum fungi. The gray domain will slowly erode all organic and inorganic objects, turning them into extensions of itself, and randomly releasing "echoes": fragments of information from lost civilizations, distorted physical phenomena, and even phantom memories of dead people.
Humanity has retreated to giant dome cities, relying on "echoers" - special groups of people who are naturally able to resonate with the gray realm - to go deep into polluted areas and collect scarce old-era technology and biological samples. The echoers' brains are regarded as the only medium that can resist the mental contamination of the gray realm, but every time they go deeper, their memories will be overwritten, contaminated, or even replaced by the echoes in the gray realm.
It is said that deep in the gray area, there is a city that has never been demarcated, which preserves the complete ancient human civilization. But all the people who claimed to have seen it never came back - or those who came back were no longer the same people.
