It was once a spotless dome, a breathtakingly beautiful backdrop for family photos and annual pilgrimages to the cherry blossoms. But dark slime and grime now covers the top of the Jefferson Memorial, an eyesore the National Park Service is ready to confront - with lasers.

What looks like soot and dirt accumulating from decades of nearby automobile traffic is actually black bacteria. The slime, known as biofilm, can't be scrubbed off. And until this summer, it wasn't clear what was the best approach to stop it.

After a year of testing different chemical compounds used to clean other architectural landmarks, the National Park Service said a laser treatment proved to be most effective, and would not damage or corrode the stone.

A contractor has been selected to perform the monumental task of removing the slime, and work is set to begin later this summer. Park officials said the restoration effort will not affect the public's ability to access the memorial.

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"There are indications a laser treatment cleans much more deeply than any chemical we've tested," said Mike Litterst with the National Park Service in an interview. "A chemical may just take off the surface of the biofilm, but not get to the root of it."

The slime first became apparent in 2009. It remains unknown why the bacteria's growth took off in recent years, or how to effectively prevent it from developing once again.

"Those remain things we're studying as part of the project to get rid of it here at the Jefferson Memorial," Litterst said. "The organism looks for rough surfaces, and after years of erosion, the memorial's surfaces have become a perfect environment for the organism to live."

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Once the laser treatment is applied, the slime dissolves into the air. Costs and exact timeframes were not immediately available, as park rangers will meet with the contractor in the next few weeks to finalize details.