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The Story Behind That Subway Sludge

The Story Behind That Subway Sludge
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Ever wonder what that black gunk dripping from the ceiling of your subway station is?

Subway platforms – particularly on the bombed-out N line – are often sprinkled with massive, black Rorschach blotches of goo, that can be traced directly to tiny stalactites emerging from cracks in the ceiling. It is a little unsettling to have mysterious sludge dripping above on your morning commute, but what exactly is the substance? And more importantly, is it toxic?

Spoiler: It’s molten tar. We reached out to MTA spokesperson Kevin Ortiz, who assured us that it is “completely harmless.”

“Tar or mastic is used to seal and waterproof the tunnel/station structures. When the mastic warms up, especially in summer, it can start to flow and drip down onto the platforms below,” said Ortiz.

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Slate’s Mark Vanhoenacker investigated the black stuff last year and got a similar response from an MTA representative. According to his findings, all it takes to clean the gunk is a scraper and a good power wash.

It turns out the MTA stopped using the substance several years ago in favor of a sealant which is less likely to drip.

Also, don’t worry about goop dripping on anyone’s head, as tar is notoriously sluggish. Remember this tar drip experiment that lasted 69 years?