It is the work street artist EVOL did for music and art festival MS Dockville in Hamburg, Germany. At first he thought it would be a challenge because there were just grass there and nothing else, so he thought why not just grab a shuffle and make an underground city. After 8 hard-working days his installation turned out to be rooftop-gardens of tall buildings that are in a way built into the ground. The tunnels between each building has room for an average person to pass by. :)