Well. Good to see you all again. I'm here late, but on the other hand I got to read this issue while sitting in the shadow of this structure, so I think it was worth it. As expected, David Uzumeri and his Legion of Super-Observant Commenters got to this issue first, so an awful lot of what appears below is cribbed directly from that site. I make no particular claims for originality.
As Jeff O'Boyle noted in the comments over at Funnybook Babylon, the Dark Monitor's pose here is based on the cover of JUSTICE LEAGUE OF AMERICA #96. Interesting! Note that Superman is wearing some kind of armor (check out the gauntlets, which are like the ones on the Monitors' "weapon" later this issue), and that his face doesn't quite look like his own, although the cape design is our Kal-El's. (We don't know who's inside the armor, of course, and Tom suggests in the comments that this might be a ZENITH-style switcheroo.)
I could swear I've seen the line "what shall we engrave upon your tombstone?" somewhere before, but I can't think of where.
This (and the opening of the next two-page spread) are pretty clearly a nod to the old DC splash-page tradition--the way stories would open with an image that would somehow encapsulate the central conflict of the story, then jump back to reveal how we got there.
A flashback to FC #3, of course. Scott over at Polite Dissent has a few things to say about the quasi-medicine on display here. Also, yeah, you'd think they'd have brought in the Purple Ray or some Kilowog-tech or something.
Is it me, or is the "recruit the greatest super-champions of the multiverse" business nearly identical to the premise of COUNTDOWN: ARENA?
"Universe designate zero": apparently DC UNIVERSE 0 was named after the place where it's set! The 52 parallel universes are numbered 0 through 51, not 1 through 52... but then what's Earth-1?
"Ultima Thule" basically means more north than north. There's also a Henry Wadsworth Longfellow poem by that title, which may be sort of relevant here.
The word "ultramenstruum" first appeared in THE INVISIBLES #22 in 1999; until now, I'm pretty sure, it was a hapax legomenon. "The Bleed" was first mentioned in STORMWATCH #7 in 1998. (Apparently Überfraulein's mention of the bleeding skies in FC #3 really was meant to have the implication of menstruation after all.) "The bleed" is also, of course, the space off the edge of the comics page
The phrase "germ-worlds" probably first appeared in Zadel Barnes Gustafson's 1879 poem "William Cullen Bryant."
"4-D vision" is also a callback to THE INVISIBLES, a surprising amount of which is concerned with the question of how to represent a greater-than-three-dimensional construct on a two-dimensional picture plane (like a comic book page); a 3-D comic book makes it easier to represent a four-dimensional construct. But what is the fourth dimension, you ask? It's time, of course; 4-D vision allows one to perceive multiple times simultaneously, instead of a temporal cross-section.
"Cast off! Weigh anchor!": I'm amused by the way Morrison has alien characters speak in totally Earthly metaphors (e.g. "dust for radiation").
As David Uzumeri notes, Übermensch is saying "We will have to accept losses! This machine is about to explode!" We first saw the Dr. Manhattan-ish version of Captain Atom in Nix's drawings in FC #2, pg. 9. And the Captain Marvel we're seeing here is not the DCU (0) one but the one from Earth-5 (a variation on Earth-S), where all the Fawcett characters live in their original incarnations.
Ultraman here--whose "weapons" appear to include a version of Etrigan the Demon and a version of, maybe, Batwoman?--appeared in his initial form in JUSTICE LEAGUE OF AMERICA #29 in 1964. Per pg. 18, this version is the one from Morrison's JLA: EARTH 2, which was actually not about the Silver Age Earth-Two (or the one Geoff Johns used recently in JUSTICE SOCIETY OF AMERICA ANNUAL #1) but about an antimatter universe that doesn't seem to be counted among the 52 basic universes. Don't confuse him with the Ultraman from Earth-3's Crime Society of America as seen in COUNTDOWN PRESENTS THE SEARCH FOR RAY PALMER: CRIME SOCIETY #1, although he is probably the Ultraman who turned up in THE BRAVE AND THE BOLD #11.
Earth-6 here, first seen for real in COUNTDOWN: ARENA #2, has a very familiar kind of New York cityscape, and is going through some kind of Civil War/Secret Invasion mashup. Plus: the guy in armor is secretly a shape-changing alien!
I like this little transtion from the plebeian quasi-Marvel Universe cityscape to the curvy high-tech one in the more brightly-lit world.
Earth-51 would appear to be the one from "moving part 51" that had been lost as of FC #1; its entire universe was destroyed in COUNTDOWN #14, then reinstated in COUNTDOWN #9, then turned into a Kamandi-like mess via the Morticoccus virus, and now... everything's dead again?
Lady Blackhawk and a character Morrison has referred to as "Doc Fate" are here on Earth-20. Earth-17 appeared very briefly in 52 #52; it's rather like the old Atomic Knights stories.
A yottabyte is one septillion bytes. The Zillo Valla/Overman scene is as weird as seductions get...
Overman is saying "All these universes vibrate at different frequencies."
Captain Allen Adam, the Dr. Manhattan/Captain Atom type from Earth-4 (established in 52 #52 as the Charlton-heroes-via-Watchmen world), is I don't think quite the same as the Earth-4 Captain Atom seen in COUNTDOWN: ARENA #4. (There was an albino Dr. Manhattan sort, Doctor Metropolis, who appeared there, according to Keith Champagne.)
And now we get to see Grant Morrison doing his own version of ARCHITECTURE & MORTALITY! This version of Limbo first appeared in ANIMAL MAN #25--see also the "monkey with a typewriter" business below. In the lower panel, besides Merryman (of the Inferior Five; created by E. Nelson Bridwell and Joe Orlando, first appeared in SHOWCASE #62 in 1966), Chris Eckert and David Uzumeri have identified Nightblade, members of the Alliance, Ace the Bathound, Gunfire, Voiceover, Ballistic, Golem, Geist, Hardhat, Chronos and Private Eyes.
Overman is saying something like "What is that? I can't remember why I made this. The whole business is for the dogs." The broken-off shard from the Rock of Eternity happened in DAY OF VENGEANCE, i.e. it may not have happened to this version of Captain Marvel.
I'm betting what the library gate should say (slightly misspelled) is "facilis descensus Averno" (or "Averni"): a line from the Aeneid, basically meaning "the road to evil is easy."
The monkey/infinite pages/infinite content business was referred to in ANIMAL MAN #25, but it goes back at least to 1913. Before J.L. Borges wrote "The Library of Babel" in 1941, he wrote an essay called "The Total Library" in 1939 that covered similar territory; the concept also has ties to Borges' "The Aleph," from 1949, and the Book of Destiny that turned up in THE BRAVE AND THE BOLD.
Of course, if the Library contains all possible books, it also contains infinite sets of false instructions for repairing the Ultima Thule...
"In the beginning" is how a story with a finite starting point begins; "Previously!" is how an installment in a serial begins. And this is, after all, a serialized multiverse. The "concept to contain the flaw" looks a bit like Metron's chair, doesn't it? Also, emphasizing "intricate" in the final panel is a very Kirby-ish gesture.
"Liiving" is not to be confused with Liviiing. The last panel is, as David U. notes, a bit from CRISIS ON INFINITE EARTHS.
The majestic golden Superman here might be a callback to the conclusion of DC ONE MILLION. I suspect that Superman-as-a-concept is the original "flaw" in "Monitor perfection," the thing that makes possible infinite stories; it's sealed over with the divine golden metal (it's not likely that the golden metal of the time travelers in that totally baffling, apropos-of-nothing scene in 52 #27, which also involved time being frozen and space between seconds, is related, but I can always hope).
So wait, is the decadent doomed civilization threatened by "loathing and greed beyond measure" and waitin' for a Superman supposed to be comics readership?
"Who knew the day of holocaust would come again!": a callback to the first page of JUSTICE LEAGUE OF AMERICA #96 (Jeff O'Boyle pointed this out).
More prophetic language I could swear I've seen before: "ultimate good is ultimate evil," "the thing most despised will save the most beloved."
Overman's cousin must've been the late Überfraulein. "Carriers, destroyers, tankers and explorers": the Carrier from THE AUTHORITY is also part of the Monitor nanotech fleet.
The book contains all possible stories, including, I'd imagine, ones where evil doesn't win in the end... and I would feel much better about that "coming soon" if SUPERMAN BEYOND #2 were actually on the schedule.
Still catching up. More to follow...