Social media site Foursquare allows users to 'check in' from their mobile device and computer from anywhere in the world.
It's supposed to be a way to share recommendations about your local area, and find out new info from other users.
But now it might have found an even better reason to exist: making awesome maps.
The service has just released the last three months of its check-in data in the form of maps, which show off the incredible structure of cities, towns and villages around the world based on how often people have checked-in.
These 500,000,000 points of data are not only beautiful, they're also informative about where people are using the internet (and Foursquare) and where they aren't.
We've used their maps to make a quiz - can you guess the city from the shape of its Foursquare check-ins?
The clock will start as soon as you submit your first answer.