In 1872, an act of Congress transformed newly acquired territory in the American west into Yellowstone National Park. The act declared that the land was "hereby reserved and withdrawn from settlement, occupancy, or sale under the laws of the United States...and set aside as a public park or pleasuring ground for the benefit and enjoyment of the people."And while it was our first national park, Yellowstone draws on much older thinking about sanctuaries. We often use the word sanctuary to talk about places like Yellowstone that have been protected from human development and industry. But it's a word with deep religious roots. Traditionally, a sanctuary is a place that is set apart from daily human life and reserved for the divine.Last winter, we traveled to Yellowstone to explore what happens when a religious idea like sanctuary is transformed into a secular and bureaucratic one. How has "setting aside" this land affected the people and animals who have historically lived within its borders? And can any man-made border keep out the effects of climate change?