What Is This Black Slime Spreading All Over Washington D.C.?


Robin Andrews

Science & Policy Writer

Can you seen the black gunk on the dome of the Jefferson Memorial? National Park Service

There’s a weird black slime coating some of the most famous monuments in Washington D.C., and it’s spreading all over the capital. Is it some sort of extremely slow alien invasion? No, of course not – this dark-hued material, which adorns the statute of Abraham Lincoln, the Jefferson Memorial, and gravestones at Arlington Cemetery, is actually collections of microbial life.

As reported by the National Park Service, this black slime – which is actually more of a grimy powder – is something called a “biofilm”, a group of microorganisms which are encased in a gunk that has particularly adhesive properties. Biofilms form due to a number of reasons, including for sharing nutrition, for boosting their collective defensive abilities, or just because they have all found a good spot to rest on.


Biofilms are ubiquitous in the most literal sense. They are found in almost every type of natural of non-natural environment, from forests and glaciers to the hulls of boats and inside your fish tanks. Some of the oldest evidence for life on Earth was found in the form of 3.5-billion-year-old biofilm layers intermixed with marine sediments. These curious structures, which are named stromatolites, still exist today.

So why on Earth are biofilms appearing on some of the most famous Washington D.C. monuments? Well, as it so happens, these biofilms can be found on monuments all across the world, from Angkor Wat in Cambodia to ancient Egyptian superstructures.

As with the Jefferson Memorial, these pieces of architecture have become eroded over time through acidic rain, which has created little pits all across their outer surfaces. These pits provide microorganisms with the perfect opportunity to settle in and replicate, enjoying the nutrients trapped in these pits for long periods of time.

Although biofilm became visible to the naked eye in 2006, it’s become increasingly pronounced in the last few years, and a group of interdisciplinary researchers are currently trying to work out how to deal with it.


“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, said in a statement.

A few methods are currently being trialed on small patches at the base of the Jefferson Memorial, and the researchers will keep an eye on their progress in the next week or so. 

Far from being a “mysterious slime”, then, these biofilms are related to the most primitive organisms on Earth. Clearly, it doesn’t matter if humans get in the way – they will always find places to colonize.

Life, as always, finds a way.


You're looking a little grimy there, Abe. Tanarch/Shutterstock


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