This is a Minecraft 1.12.2 server running the Lost Cities
map generator mod. Automatically generated abandoned city-scape stretches in all directions, with densely packed cities near spawn and more wilderness further out. (Current map boundary is at 10km from spawn.) This includes highways, subways, railways, and an impressive variety of buildings. Hidden throughout the buildings are loot chests and mob spawners, making the world essentially a giant dungeon.