Less than two miles (3 km) from where triggers for thermonuclear weapons were once manufactured and against the backdrop of Colorado's Rocky Mountains, a bull elk bugles as he defends his harem of cows from rival males.
"It's rutting season and this is mating behavior," said David Lucas of the U.S. Fish & Wildlife Service, and manager of Rocky Flats National Wildlife Refuge, 12 miles (19 km) northwest of Denver.
He was escorting a Reuters journalist on one of the first official tours of the 5,237-acre (2,119-hectare )Rocky Flats National Wildlife Refuge since it opened on Sept 15.
The unique prairie ecosystem is home to 239 wildlife species, ranging from elk and mule deer to black bears, cougars, numerous bird species, and monarch butterflies, Lucas said.
It's unclear how long the refuge will remain open.
Five environmental and community activist groups have sued the government, arguing the Rocky Flats refuge should be closed until more testing is done. A judge last month rejected their request to delay the opening while the lawsuit is heard.
The suit is pending in Denver federal court.
The Rocky Flats plutonium plant had a history of fires, and radioactive spills during its 37 years in operation before shutting down permanently in 1989 during a criminal investigation into environmental violations.
Now that it's a refuge, its 10.2 miles (16.4 km) of trails are open to naturalists, hikers, cyclists, and equestrians. About 1,300 acres (526 hectares) immediately surrounding the old production facility is permanently closed off to the public.
Because the weapons production was a classified defense operation, no development was allowed on the site so the area has been untouched by human encroachment, leaving it in pristine condition, Lucas said.
In the lawsuit, pending before U.S. District Judge Philip Brimmer, opponents of the refuge argue that a $7.7 billion Superfund cleanup overseen by the Environmental Protection Agency was flawed and a new environmental study should be conducted.
"Our case will clearly demonstrate that the government does not have an up-to-date assessment of risks to the environment and human health from allowing unlimited public visits to the refuge," Randall Weiner, an attorney for the coalition, told Reuters.
Both the EPA and Colorado health officials have determined that the site is safe and background radiation is within acceptable limits.
"This is one of the most studied pieces of land on the planet," said Lindsay Masters, an environmental protection specialist with the Colorado Department of Public Health and Environment, noting that several studies of the soil, water and air have found that the site is not a public health hazard.
Rocky Flat's demise began in 1989 when FBI agents and EPA investigators raided the plant based on a whistleblower's tip that contractors were illegally disposing of hazardous materials.
The contractor at the time, Rockwell International Corp., pleaded guilty to violating environmental laws and paid $18.5 million in fines. In 2015, Rockwell and the plant's previous contractor, Dow Chemical Co., paid $375 million to 12,000 homeowners downwind from the plant after a federal jury found the companies were liable for devaluing their properties due to plutonium releases. It is unknown when Judge Brimmer will rule on the current lawsuit, but it may not end the controversy. The losing party could appeal and a lawsuit filed by the nearby town of Superior seeking to close the refuge is in its early stages.