The steep walls of the Grand Canyon, carved out naturally over time by the Colorado River, reveal 19 distinct physiographic sections of rock from the Colorado Plateau. Located in Arizona, the canyon is one of the seven Natural Wonders of the World, and has a history of human inhabitance dating back to at least 1200 BCE. From the rim to the bottom of the gorge, the colorful striations of different rock layers tell the geological history of the canyon and how it has changed, and grown, over time. At the visitor center, you can learn more about the indigenous people that first made the canyon their home, as well as their descendants who still populate the area in small, remote villages along the river valley. If you’re extremely adventurous (and fit), you can hike 8 miles into the village of Supai, one of two places in the US—the other being Phantom Ranch, further up the canyon—where mail is still carried in and out by mule trains.