Abandoned (mostly) bomb shelter - Beijing.
During the Cold War, the largest bomb shelter in the world was commissioned beneath the streets of Beijing. Intended to house 6 million people, they not only wanted their citizens to survive a nuclear attack, they wanted to thrive and survive underground if such an event happened.
The tunnels were never finished and a recent explorer found an entrance and discovered a map down there (second photo).
While walking through, he flicked a lightswitch and discovered that the whole place still had electricity running right through it.
Finally, he stumbled upon a homeless family who had set up home down there and even had a fully functioning kitchen (last photo).