The Cuckoo Cries

Driven to the brink of extinction by a colossal tidal wave and the deadly miasma that followed, the remnants of humanity have fled the toxic surface. To survive, they now build their cities atop the colossal bodies of giants. Left behind in the poisonous depths below, a lower-class boy makes a startling discovery: his master's son looks exactly like him... (Source: Naver, translated)