It immediately expands the city's borders by one tile in every direction if those tiles are not part of another city (either your own city of an enemy tribe's). It will also explore a 5x5 area around your city, regardless of the tile being in enemy territory or not.
You get +20 points for every new tile in your city.
|Ruins, Population, and City Upgrades|
|Ruins, Super Unit, Battleship, Technology, Explorer, Population Growth, Population, Giant, Park, City Wall, Workshop, Border Growth, Resources|