On the second time you run though dead city it just occurred to me anyone notice the red gooey substance has grown in size or expanded slightly I can't remember it being on the middle floor with bits stretching and branching out just curious if anyone can confirm my suspicion post it, may be a clue for future dlc or something as it's not really explained in the campaign to what it is. Cheers

That is why most are calling this a Tech Demo for Doom 4. It was pretty blatant, with the Doom sigils as graffiti on the same level. In fact, it bursts through walls and ceilings right in front of you. So it was growing.