DENVER (AP) — From a tiny Pacific island to a leafy Indiana forest, a handful of sites where the United States manufactured and tested some of the most lethal weapons known to humankind are now peaceful havens for wildlife.

An astonishing array of animals and habitats flourished at six former weapons complexes — mostly for nuclear and chemical arms — because the public and other intrusions were banned for decades.

When they became obsolete, the government converted them into refuges under U.S. Fish and Wildlife Service management, but the cost is staggering. The military, the U.S. Department o...