The Yetman brothers continue their trip of exploration through the Grand Canyon, this time focusing on the geology of the canyon and the desert vegetation found along the river. They go plunging through Lava Falls, one of the last big rapids on the Colorado River and watch adults of all ages turn into sprightly kids again while jumping from the edge of a waterfall into a pool below. Two great mysteries about the Grand Canyon are explored in this episode. First, there is a geological period of time that is unaccounted for in the history of the canyon. What happened during this time and why is there no evidence of it? Second, what happened to everything that was in the Grand Canyon? Did it get washed out into the Sea of Cortez?