The 704 Project is one of Hangzhou's more unusual modern-history sights: an underground bunker complex built during the tense early 1970s and associated with Lin Biao. It is a sharp change of mood from West Lake, temples, and tea fields, which is exactly why it is worth considering.
The site is best for travellers interested in twentieth-century Chinese history. Expect tunnels, utilitarian rooms, and a story about fear, secrecy, and political survival rather than decorative beauty. It will not be everyone's Hangzhou, but it broadens the city's timeline.