"My wife and I like to travel, camp, and hike. On one of our favorite trips to the Southwest we camped at Mesa Verde (Spanish for Green Table) and visited the Pueblo cliff dwellings, which I had thought were built for fortification until their mysterious abandonment. In reality, the dwellings had a practical design – it was easier to carry food down the cliff from the lush mesa above than to carry heavy water up the cliff. This arrangement worked so well that the Pueblo population grew until they exhausted the mesa’s resources. Experiencing this was a real light bulb moment, while also serving as a cautionary tale for sustainability."