Abraham Lincoln and Thomas Jefferson have been slimed.

A mysterious black slime has been steadily oozing over Washington DC's most famous monuments, including the Lincoln and Jefferson Memorials.

The Jefferson Memorial's white marble dome is slowly turning gray from the icky substance, which is called biofilm, and not actually a slime, but more of a powder, reports CBS News.

The black stuff has also creeped over the Arlington National Cemetery and on the Washington Monument and the tombstones at Congressional Cemetery, according to NPS.gov.

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The normally pure white marble dome of the Jefferson Memorial has turned gray-ish with biofilm, otherwise known as black slime (seen above)

The microorganisms can look like dirt, which was causing the public to complaint that the parks department wasn't cleaning the sights well enough

The parks department finally went public with their slime problem and are still seeking a solution

'It’s called slime, but when you touch it, it’s dry; it doesn’t come off on your fingers,' noted CBS News’ Mark Albert when he visited the area.

Because of its dryness, it's even more difficult to clean off, said Mike Litterst of the National Park Service.

All biofilm, which is a colony of microscopic organisms, really needs to grow is nutrients and a surface such as stone. Many famous monuments all over the world have the same issue.

Rain can cause pits to form in smooth stone surfaces over time, creating the perfect petrie dish for the slime.

But getting rid of it isn't as easy as soap and water.

'Treatment of biofilm is difficult, as there is no known permanent method for removing it, and we have to ensure that any treatment must not do further damage to the soft marble of the memorial nor encourage further growth,' Catherine Dewey, chief of resource management for National Mall and Memorial Parks, told NPS.

The slime has creeped from the National Mall to the Arlington National Cemetery (above) and the tombstones at Congressional Cemetery

The parks service is testing a variety of treatments to find one least damaging to the stone, is safe for the environment and visitors, and isn't too expensive.

The department has actually had to hire a team of global scientists to attack the problem.

'It is very frustrating. And it’s one of those things the more we look into it, the more questions we ask, the more we come up with all sorts of theories and ideas, but to date there is no known permanent solution to get rid of biofilm,' Litterst said.

Although the slime problem has been going on since at least 2006, it has rapidly accelerated lately - and the public noticed, complaining that the city wasn't doing enough to keep the shrines clean.

That's when the department went public with its dirty biofilm secret.

But Litterst is determined to fight back.