Rocky Mountain Arsenal National Wildlife Refuge sits just northeast of Denver. Inside the refuge's 16,000 acres, bison roam and prairie dogs scamper in view of a professional soccer league stadium and the Denver skyline.
The refuge opened the doors to its visitor center in 2011. Since then, visitors numbers have skyrocketed, said David Lucas, who manages the refuge for the Fish and Wildlife Service.
"Last year we had over 300,000 visitors."
Before long, he expects that number to reach a million a year. In response, the refuge is putting together a plan to open new areas to the public and continue improving the habitat for wildlife.