In the foothills of Boulder County, Colorado, there’s a kind of secret water park. It’s a sprawling network of pools, channels and waterfalls. Neck-deep ponds, rushing streams and cascades twice the height of a person are a stark contrast to the dry, brushy terrain on the canyon slopes above. But these features weren’t built by humans. This marshy mosaic is a paradise for beavers, and one of hundreds of thousands just like it across the American West. The animals create messy wetlands as safe places to live, and a new paper explains how their handiwork is also a powerful tool in fending off the harms of climate change.