Sam Schwartz

Do you ever wonder what that black schmutz that collects on the subway platform is? Has all the discarded chewing gum in the world begun to mobilize? What if I put my bag on it? Slate's What's That Thing column appropriately examines the phenomenon: After repeated unreturned phone calls to the MTA, the intrepid journalists got "Gridlock Sam" Schwartz and his transit gurus on the case. Finally the fine folks from the MTA rallied and gave name to the mysterious muck. According to NYC Transit assistant chief of the Division of Stations Branko Kleva, the stuff is mastic, a tar-like substance used to seal and waterproof the subway tunnels. When it heats up, (in the summer, for example, ironically from the heat generated by subway car air conditioning) it starts to "flow and drip down from the roof of the tunnel onto the platforms below."
More on the mysterious gunk...