But one county site on the National Register of Historic Places is missing: the 1-million-liter test sphere, one of the largest relics from the days when the Army not only made vaccines against biological warfare, it was also developing biological weapons of its own.

"It’s certainly something unusual that we have in the county," said John Fieseler, executive director of the Frederick Visitors’ Center.

That uniqueness has not made an easy sell of the "eight ball," as the 40- foot-high, gas-tight globe at Fort Detrick is known. Fieseler said no one has ever stopped by the visitors’ center to ask about the test sphere and he doubts very many Frederick residents even know it’s there.

Officials at Fort Detrick estimate that 500 people—largely former base employees, scientists and State Department and NATO visitors—have visited the eight ball within the past year.

But even they concede that "the average soldier at Fort Detrick, generally speaking, has little idea what the eight ball is or was," which is why the history of the three-story globe is now required as part of soldiers’ required knowledge of the fort.

In its heyday, such ignorance about the eight ball would have been unimaginable.

From 1951 to 1969, when President Nixon signed an executive order ending the nation’s offensive biological warfare program, scientists at Fort Detrick used the gas-tight chamber to help develop biological weapons for the United States. The sphere was also used in the development of vaccines against other nations’ biological weapons.

Wearing rubber gloves attached to control panels at the base of the eight ball, scientists would open hermetically sealed cans of biological agents inside the globe’s inner chamber. They then used detection devices built into the inner chamber to track the travel speeds and dispersion rates of different aerosols.

As part of the Army’s offensive weapons program, scientists detonated biological munitions inside the sphere and tested different ways to disseminate their deadly agents—from bombs to sprays.

Scientists determined lethal doses of biological agents by exposing animals to them, according to base officials.

They used both humans and animals in the Army’s defensive research program -- scientists first used animals and then human volunteers to test vaccines they had developed against biological agents such as Q Fever. The volunteers, often drawn from the ranks of conscientious objectors, would be vaccinated and then step up to a portal in the sphere to suck up a lung full of germ-laden air.

The eight ball was the only one of six Army-operated test spheres in the country equipped for human volunteers.

On the 1976 application to add the eight ball to the National Register of Historic Places, Fort Detrick officials said that "as far as is known, the 1- million-liter test sphere is unique—the largest such facility in the world."

But Nixon’s 1969 executive order and the subsequent Bioweapons Accord of 1975 closed out the United States’ era of offensive biological warfare research and human testing.

In 1975, a fire destroyed the 60-by-60-foot lab building that enclosed the eight ball. The blaze destroyed control panels and exposure chambers at the base of the test sphere, as well as a catwalk that let scientists observe exploding biological weapons from portals at the globe’s equator.

What’s left today is a dull gray sphere that "just sits there and rusts," in the words of the fort’s public affairs specialist, Chuck Dasey. When visitors see it now, he said, "The first question usually is,  ‘How did it work?’"

Dasey says the eight ball now serves as "a visual reminder of the past," a time when certain labs on base were guarded by submachine gun towers and scientists left pistols handy on their workbenches, according to a history of Fort Detrick by camp historian Norm Covert.

It is a history, once cloaked in secrecy, that base officials now hope to lure the public to see. They have begun to work in conjunction with the Frederick Visitors’ Center on a walking tour of the fort, featuring the eight ball and several buildings that are on the register.

To get to the sphere, visitors have to walk through an opening between research labs, which have been built up around it over the years and almost completely surround the structure today.

Despite its size, the sphere is unremarkable next to the building it abuts. There are no indicators of its past importance except a historic marker affixed to a back leg of the sphere, facing the building.

Base officials said they do little maintenance beyond the occasional paint job and picking up litter around the sphere. Covert, the base historian, acknowledges that the sphere is a white elephant. But he also sees beauty in the relic.

The walking tour of the fort is already in place, and officials hope to develop a driving tour as part of their effort to show that Fort Detrick contributes to the community and is more than a "faceless headquarters."