But the apple gets Peter thinking about the "amazing apple pie" he and Olivia had "just after the invasion" in Boston. He prattles on obliviously about the dingy little restaurant they found with the "mad genius" cook while they were looking for a place to eat after going to the shelter for lost children. "What was that guy's name?" Peter asks, and a CLEARLY UNCOMFORTABLE Olivia is all, "I don't know," and Peter can't believe that Ms. Photographic Memory can't remember and fails to clue in to Olivia's obvious discomfort, not to mention it doesn't occur to him that it's really, really, REALLY weird to be fondly reminiscing about a time when they had just been to a shelter for lost children to look for their DAUGHTER WHO WENT MISSING DURING AN INVASION OF PSEUDO-OMNISCIENT NON-HUMANS, I mean JESUS CHRIST PETER WOULD YOU LISTEN TO YOURSELF. I mean, it looks like even Etta, who basically met her mom like two days ago seems to be able to tell that something's up.
Anyway, what they're doing is an air sample analysis and Walter looks at the red color that the test strip turned when they were DOING SCIENCE and says the man died of asphyxiation: "Every pore on his body is completely sealed. His body was trying to protect itself against the airborne corrosive," he explains. The corpse's pH has been radically altered and it seems to get much worse the deeper you go into the mine. Walter doesn't seem to notice everyone's squeamishness when he casually yanks the crusty arm off the corpse.
Peter asks if what they're looking for could have caused this, and Walter says it's unlikely, but his theory is that it's the high levels of carbon dioxide and other gases introduced into the atmosphere by the Observers. "From those machines that they were building in Central Park," says Peter. Olivia wants to know how that would affect Edwin and the treepeople, and Walter says they're far enough away from the source and receive only incremental doses. But prolonged exposure has created a "drastic immune response," like an extreme form of psoriasis. I'm not quite following this -- there may be something that I'm missing, but it's that there's something in the mind that's reacting to the carbon dioxide and other gases, right? Wait, I keep forgetting it doesn't matter. Anyway, Edwin shows up to say he found an entry that he thinks Walter will want to see.
Down in the archive, the Fringe team checks out an entry that notes that five weeks after the invasion, a man came to the camp and went down into the mine and gathered several rocks with a reddish hue. There's a picture of the guy from behind, and despite the fact the guy has, you know, HAIR, Walter asks if the man was named September. Edwin says his father recorded it as a man in his late 30s with dark hair named Donald. "He says he was waiting for a scientist from Boston. I was thinking perhaps that was you," says Edwin. So just to be clear, these treepeople are saving news articles from around the world but also writing up stuff that happens around the camp, right? The Treepeople Times must have a goddamn thrill on every page.