On Thursday night's Fringe, called "What Lies Below," we were introduced to an ancient supervirus - and learned Walter's darkest secret.

This episode was clearly a tip of the hat to X-Files. It was about an ancient virus discovered in oil, and oil infection imagery was everywhere. (For those who missed out, one of the major plot arcs on 1990s paranormal series X-Files was a disease/conspiracy thing that manifested as a swirling black oil in people's eyes.) Apparently some oil company had extricated a chunk of rock from 10 miles below the Earth's surface, and viruses from the chunk got released in an office building due to some corporate espionage plot gone awry.

Olivia and the gang are called in when people start dying with red virus mist coming out of their mouths. There's a whole awesome mad scientist explanation - mostly provided by Walter - about how the virus can take control of people's minds and make them do anything to get the virus into other people's bodies. Things like jumping out the window of the quarantined office building to spray virus goo on the CDC, or grabbing a gun and trying to shoot Olivia (which is what the infected Peter does). Walter freaks out over Peter's infection, then is inspired: Maybe the virus is what wiped out all life 75 thousand years ago (huh? maybe he meant 75 million years?) but then the explosion from Krakatoa covered the Earth in sulfur and that's what killed the virus the first time around! Quick - get some sulfur pumped into everybody as an antidote.

All of that is neither here nor there, really. The science was weird, and the action was people chasing each other around and a race against time to get an antidote before the CDC killed everybody to "contain" the threat. (Note to Fringe writers: How about a spinoff focused on a gang of CDC assassins?)

The really good bit came when Walter accidentally spilled his guts to Astrid - as you can see in the clip, he said "Peter is going to die again." In an episode last season, we glimpsed a mysterious grave with Peter's name on it, and recently we saw Walter freak out over Peter when he sees coffins - and there have been strong hints that Peter is actually an Earth-1 Peter that Walter stole when Peter from Earth-Prime died as a boy. Now at last we have our final confirmation. Walter has stated explicitly that Peter has died before. Whether that means he died temporarily and came back to life, or Peter-Prime died and our hero is actually Peter-1, OR that Walter just IMAGINES that Peter-Prime died . . . we just don't know.

But we can wonder.