To me, and I have been around those mills A LOT, it looks like the pressure built up in one of the digesters. A digester is what could be compared to a HUGE pressure cooker. It is used to break down the wood chips into fiber before it is spread out on the screens to start the papermaking process after it is all bleached out to make the paper white.
That black stuff? Thats the back liquor and its a byproduct from the wood fiber.
Black liquor is often used in the boilers in those mills for fuel and, it can be reclaimed and re-used in the pulping process.
Black liquor can also be used to make a very fine bio-fuel for engines. Except, our stupid EPA regs do not allow it-or at least did not allow it.
What is the black liquor...is it a petroleum byproduct of the cooking process, some kind of pine tar liquid?