Seven billion people is a tough number to truly comprehend. It's like geologic time, or the number of neurons in your brain. It's on a scale that we have little mental framework to understand.

We tried to gain a little perspective on this staggering total with some statistics, but sometimes the big picture is best understood as a collection of smaller scenes. National Geographic is marking the occasion with a free "7 billion" iPad app based on their yearlong series on world population, and we've got a preview of some of the images and videos in this gallery.

Above:

Sharing a hillside with high-rise apartment dwellers, children dance at a shop in one of the squatter communities that ring Caracas, a city of 3 million. One in seven people on Earth lives in slums today. Providing them with better housing and education will be one of the great challenges facing a world of 7 billion people and counting.

Photo ©Jonas Bendiksen/National Geographic