The Story Behind That Subway Sludge

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.

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?