Great natural monuments - Grand Canyon: The Grand Canyon is a breathtaking natural monument. It's one of the planet’s best-known landscapes, yet we often forget that it tells two billion years of the Earth's history. Geologists Karl Karlström and Laurie Crossey are the leading experts on this. Over the course of 8 days, they descend the Colorado river and tackle hundreds of rapids to unveil the mysteries held in the Grand Canyon, and the place that humans occupy within it.