It's black, grimy and growing, but what, exactly, is it engulfing one of the nation's most hallowed monuments?
Conservationists have been left baffled by how to stop a microbial invasion that has slowly been covering the Jefferson Memorial, causing the 73-year-old white neoclassical structure to take on a dingy look.
National Mall and Memorial Parks Chief of Resource Management Catherine Dewey says the biofilm became noticeable less than a decade ago and has grown "immensely'' in recent years.
The Park Service is experimenting with several cleaning solutions to remove the biofilm without damaging the marble. But officials aren't sure whether any of the efforts can prevent the community of microscopic organisms from coming back.
"Treatment of biofilm is difficult, as there is no known permanent method for removing it," Dewey said.
Milan-based microbiologist Federica Villa says she does not know whether the biofilm is damaging the stone.
NPS officials say the treatment that is ultimately selected will determine the timeline and the cost.
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But the dingy tinge of biofilm is not unique to the Jefferson Memorial. The National Park Service says it can be found throughout Washington.
