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What Is that Gunk on the Subway Platform? We Have an Explanation

By Savannah Cox | February 23, 2016 1:26pm
 Mastic makes an appearance at Columbus Circle.
Mastic makes an appearance at Columbus Circle.
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DNAinfo/Savannah Cox

New York City subways are filled with rats, detritus, and hideous black gunk — the latter of which is made all the more off-putting because most of us just don't know what it is.

As part of Slate's "What's That Thing" series — which seeks to examine oft-overlooked curiosities of our visual environment — the online publication set out to determine just what those angry, black splats peppering subway stops throughout the city are, and why they're there.

Their findings? The spots are tar deposits, and are the results of old ways of thinking about infrastructure protection.

NYC Transit assistant chief of the Division of Stations Branko Kleva told Slate that the mysterious black gunk is actually a material called mastic, which the city has "used to seal and waterproof the tunnel structures."

Over the summer or just when heat accumulates in the platform, Kleva said, mastic can heat up, lose its solid properties, and begin to drip "down from the roof of the tunnel onto the platforms below." Once it reaches its destination — the floor — it pools and congeals, resulting in the inky tar-like splashes that subway riders see regularly.

Sometimes, Kleva added, the heated mastic cools before dripping onto the floor and forms "mastic stalactites," or the candle wax-like streams of blackness staring down at riders while they wait for the train.

The mastic pools and stalactites might seem menacing (and they certainly sound it), but the MTA assured Slate that they're "nothing to worry about."

In fact, we shouldn't spend too much time ruing them since, well, they're probably not going anywhere any time soon.

While new subway tunnels will use "new, different kinds of sealants" that are less likely to yield the black gunk, Kleva said, the reality is that a comprehensive resealing of older tunnels is "prohibitively expensive," meaning that the mastic sealant is likely to stay for a while.

How to get rid of them without installing a completely new waterproofing system, then? For now, Kleva says, all we can do is "scrape them off, or powerwash them."