It sounds like the stuff of horror films: a creeping black slime that can't be killed. But experts say a real-life microbial invasion is coating some of the nation's most important monuments in black. The National Park Service earlier this month reported the "biofilm" has befouled the Jefferson Memorial, particularly its "gleaming white rotunda," and appears on the Washington and Lincoln Memorials and on tombstones in the Congressional Cemetery. But the mysterious substance—"part algae, part bacteria, part fungi," the Washington Post reports—isn’t unique to the Washington, DC, area. Biofilm has slimed sites around the globe, from Egypt to Italy to the Angkor Wat temples in Cambodia, per the NPS. Scientists are only now "starting to understand what it is, and its relationship to stone," a senior conservator with the NPS tells the Post.
The NPS reports it has uncovered a "common factor" related to the establishment of various biofilms: the somewhat vague "presence of nutrients and a place to grow, like stone." It suspects that in the case of the Jefferson Memorial, the dimples and holes that have formed in the eroding marble may create an environment ripe for biofilm. That soft marble poses an additional challenge, notes the NPS, as any treatment needs to be tested to make sure it won't actually inflict harm on the stone. While there's no known permanent removal method, the NPS is evaluating 10 chemicals as potential cleaners and intends to also review options like ozonated water and lasers. The Jefferson Memorial's biofilm first became readily visible in 2006, and there's currently no timeline for treatment.