When I was a kid growing up in Chicago, I used to fantasize that the mastodons whose bones lay deep under the pavement would rise out of the earth and rampage down Lake Shore Drive. Decades later, I began reporting on a group of quirky scientists who were reconstructing the lost world of the mastodon in remarkable detail-using everything from prehistoric poop to the DNA preserved in ancient fur. Most of them shared my dreams of snuffling, roaring Ice Age giants, and a few believed they could really bring that world back to life. I had to write about this.