Dr Allan Maca explores a newly discovered ancient megacity that 4000 years ago was one of the biggest on the planet. Its people created amazing Maya-like carvings like nothing ever found in China before. There's also grizzly evidence of human sacrifice on an unprecedented scale. This lost civilisation changes everything we thought we knew about the origins of China, but why, after 500 years, did they vanish? Allan and his team investigate ancient cataclysms to try to solve this great mystery.