(Woo! Isa Polygon now!) On topic - Yes; the search has been going on for a couple of years and it's very likely that it's going to continue until Team ICO reveals the secret. ... That, or we'll figure it out before then; we seem to be making progress in this respect. As for the origin of the Last Big Secret: Ueda mentioned in an interview (I don't recall when) that there was one last big secret in the game that no one had discovered. The Secret Garden, as far as I can tell, was common knowledge at this point. Another thing is that Devs, whenever questioned, work their way around the question whilst dropping subtle clues. ie: (to the question "Are there any secrets left in the game") "Not that I can talk about.
" That blasted winky smiley. Grr.
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But anyway, there's tons of material to support the existence of a LBS, not even mentioning the existence of so, so, SO many coincidences / links in the game that seem to lead somewhere. On to your post.
I. Tri-Block Theories
A. Celosia's door > Tower
B. Secret Garden Overview
C. Underground Tower. What are you referring to in this? The Shrine of Worship?
I was under the impression (for a while) that the new blocks were just cut-and-mirrored (Diagonally) versions of the original blocks. However, this doesn't hold true; the 'A' block has individual markings on either side, the foremost being a Pyramid-shaped structure with a sun / moon / big honkin' circle floating above it. I'm not looking at the photo right now, but I think there are two dots in that portion of the block as well. The only thing that this seems remotely similar to, imo, is the door / archway in Barba's prison. The circle, of course, being the opening in the ceiling. (ICO 3 screenshot, anyone?) I'm still not sure on this one.
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The B Block - This could be any number of things, but it looks to me like a top-down view of Malus' Imperial Mountain ( + credit for catching the Mario 64 reference). It even has the little piece jutting out at the corner for big boy's platform. Not really sure about this one, either.
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The C Block - Perfectly symmetrical. Oddly enough, I didn't see any little diamonds / squares in the original Design. So, obviously, this rules out the theory that they've been cut up and reassembled. Like the other two, I can't say for sure what this is a picture of, but my first impression was of a face. More specifically, Basaran (no.9)'s face. I know that his lightning-bolt attack comes from three points somewhere near his chin. We also have three lil' diamonds. What do you guys think?
- And... yeah. That's all I can think of on the blocks.
Next up, the Sunken Castle.
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II. The Sunken Castle
A. Dormin explicitly states that Hydrus is swimming over a hidden ruin. The fact that the word 'hidden' is used strikes me as very odd. If the spires jutting up from the water are any idication of size, the 'ruin' would have to be absolutely enormous. It seems so odd to me because Dormy always gives subtle clues as to how to slay the colossus in question. Ruins have absolutely nothing to do with kill Hydrus. Suspicious much?
B. I don't remember where it was, but somewhere in the Forbidden lands, you can just make out a pillar off in the middle of the ocean by its lonesome. I don't remember who told us this, or where it was, but this seems to fit the bill of 'sunken castle' quite nicely.
C. Avion's Ruins - Also quite immense. The architectural style seems very similar to that of an abbey or citadel. I've considered this as a candidate for a sunken castle, but it seems unlikely.
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So, in summary.
I. Block Theories
A. Block One - Asymmetrical. There appears to be a pyramid in the top right foreground, with a large semicircle in the background. A dot on either side of the pyramid. Also, the outline of a smaller pyramid can be found inside the first.
1.) The general idea here could be a doorway with a circle over it. This seems to be turning up quite a lot. I'm beginning to doubt is validity as a coincidence.
2.) This could be a subtle reference to balance: the coexistence of good and evil, if you will. The circle is 'balanced' on top of the pyramid, which is flanked by a light dot and a dark dot. The two spheres overlapping on the bottom-right of the block could indicate the need for both forces to coexist. This is highly unlikely, but I think it's worth mentioning.
B. Block Two - Symmetrical. This four-limbed design is vaguely reminiscent of Malus' mountain, with a protrusion at either end for the entrance platform and Malus' platform, respectively. There seems to be a gaping hole in the center.
1.) I really don't see how this couldn't be Malus' mountain. I'm not sure what the circle in the center is supposed to be, but the rest fits perfectly. Maybe the circle is a reference to the underground labyrinthine system?
C. Block Three - Seems to have a very elaborate arrow pointing to the bottom-right corner. In said corner lies a v-shaped bridge or, possibly, row of columns. Above the columns (for reference's sake) are three small diamonds which serve as the base for a long, shaft-like object. The latter comes to a point at the top-right corner of the block.
1.) This could be one of two things, imo - Either Basaran's face (Three diamonds, while Basaran's lightning attack comes from three points arranged in the same pattern), or, as proposed by Superyorda, a tower / spire. If it were the latter, I don't see the point of the three diamonds. That being said, it might fit as one of the 'hidden ruin''s spires.
II. The Sunken Castle
A. Hydrus' area - Dormin explicitly states that Hydrus is swimming over a ruin hidden in the lake. Hidden generally means 'not meant to be found'. The spires that dot the surface of the lake go deep underwater, and, considering the sheer size of the area, a castle is entirely possible. I would even go so far as to say... plausible.
1.) Again, Dormin mentioned the ruins. Why mention them if they're not going to aid in the slaying of the colossus? I really think there's a reason for this.
B. The Seaward Pillar - I really don't remember mentioned this, or when, but I do seem to recall seeing a screenshot of a pillar with a lopsided top jutting out of the ocean some ways out from shore. Any evidence or screens of this would be highly appreciated.
C. Avion's area - I doubt that this the castle, seeing as how the layout of the ruins indicate an intricate interior that seems to have been at least semi-symmetrical.
1.) Before being attacked, Avion is perched upon an enormous pillar in the center (roughly) of the area. This indicates (to me) a central support beam. If this were a sunken castle, then (considering that the ruins don't even stretch to the cliffs) Avion's abbey would be an entrance hall at best.
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.... Phew. That was a lot to type. Hope you guys enjoy. ^^