It took me a while to realize that the password that opens up the exit to the Mechanical Age in Myst is actually a representation of everything you had to do to make it there. The C is for the two C's you had to align to get the elevator working, then there's the icon representing the elevator buttons you had to figure out to access the rotation controls, then the third icon is three spikes and a circle, representing the three surrounding islands, two of which have round platforms that show you parts of the password while one has the round platform you put the password into. The last icon is a semicircle which I saw as representing the fact that you have to go back to the entrance where the semicircular gear is.
The five sounds needed in the Selenitic Age are the sounds of whistling rocks, running water, blowing wind, rumbling lava, and a broken clock. If rocks are taken to represent earth, and lava represents fire, the first four of these are the four classical elements. The clock can be seen as a fifth element of time.
In an interesting "close the circle" loop, consider the original Myst book that brought you into the Mystverse. Atrus said, "I've tried to speculate where it might have landed, but I must admit such conjecture is futile." In the beginning of Uru, as you're wandering around the hill in New Mexico, you find the remains of the scope that collapsed into the Star Fissure at the end of Riven laying on the ground. Assuming that the book landed anywhere near where the scope did, Atrus's Myst book landed less than a hundred yards from the cleft where he grew up and first learned of the Art of Writing, as described in The Book of Atrus.
If you wait long enough after the panel goes black in the bad endings where you free the brothers, there's some sound going in the background. The Red Book has some low subterranean rumbling, and the Blue Book has a harsh wind blowing with some crumbling rocks. In Myst 4, you get to see the worlds those books originally linked to, and those same sounds can be heard in a certain location. Spire's is in a corridor between one of the elevators and the Spider Throne, and Haven is around the area where the shipwreck is.
In The Path of the Shell, the old fireplace code only gives you a Relto page. Everyone knows that the way to get the new combination is by overlapping grid patterns throughout the other Ages. But there's another allusion to it in the first game. Flip 10 pages back from page 158 in the pattern book, and you'll find that the new code was right there the whole time.
Fridge Logic: Atrus clearly has access to empty books in K'veer while trapped (hence how he wrote the trap book for Gehn, oh wait, I mean the fake K'veer book that the Stranger somehow convinced Gehn to use). He's trapped in K'veer, a mansion in the ancient D'ni caverns, directly under the Cleft, which is where the fissure ends up and where his book fell. So if I were Atrus, I would have written the "Giant Drill Bit Age" or similar, then used the machinery there to drill my way out of the mansion into the caverns, then to the surface—and oh, look, there just happens to be a fully functional Myst book here, meaning Atrus isn't trapped anymore! Although to be fair, he was probably too busy keeping Riven stable so his wife wouldn't ... wait, what's the worst that could happen? She falls into the fissure? That's even better! Imagine if Atrus had tunneled out to the Cleft and found his Myst book and his wife waiting for him. Then again, by then she would have been able to collect the page from the vault and free him anyway ... but then she wouldn't find him there because he would already be tunneling out of the caverns, meaning they'd chase each other in circles around Myst and D'ni, eventually find each other, burn the red and blue books, and call it a day. The Moiety meanwhile would have all fallen into New Mexico and assassinated Gehn, unless he was in 233 at the time, in which case he'd be trapped there, which is even better. Of course, then there would be nobody around to link into J'nanin once Saavedro stole the Releeshahn book, unless they had a Moiety man over to visit at the time. But yeah, the Stranger seems pretty extraneous when you think about it.
The worst thing that could happen if Atrus stopped trying to balance Riven is that Riven could have destabilized badly enough to "unlink" from the Reference Book (which it eventually did), either rendering it unstable enough that it would be unsurvivable or permanently breaking the link, separating Catherine and Atrus forever since no known links between Riven and D'ni existed in Riven or accessible contact Ages. If such a link existed, Gehn could have used it a long time ago to return to D'ni, which would break the plot point that Gehn would use the trap book because he had no other way back to the city, and neither Atrus nor Catherine nor Gehn knew about the Star Fissure link. The fact that Atrus "tried to speculate where it might have landed" when his Myst book fell into the fissure indicates that he doesn't know that the Star Fissure leads to the Cleft or even if a fall into it was survivable, and so he'd have no way to know until after the events of Riven that such a thing was possible (the Stranger having survived the fall when Riven collapsed and then returned), at which point it was moot.
And as for "Giant Drill Bit Age", that is essentially what Atrus does at the beginning of the Book of D'ni; he Writes Averone with the intent of getting more manpower with which to break out of K'veer and find any more survivors.
The Selenitic Age is the only one in which Sirrus and Achenar don't have rooms, and at first glance the placement of the red and blue pages appears to be fairly arbitrary; however, it was later echoed in the nature of their respective prisons in Revelation. Achenar's page is in the 'Oasis', an area with trees and running water, and his prison age, Haven, is a lush desert island. Sirrus' page is in the Crystal Forest, and his prison age, Spire, is made entirely out of crystal.
Not only that, the reason why Sirrus and Achenar didn't keep rooms on Selenitic Age simply because there weren't any people there to dominate, so they didn't care about it.
I thought all those bedrooms were simply vacation homes that Atrus built for them when they were growing up, and Selentic had none because the age was unstable to begin with, and aside from the Scenery Porn there was really no reason to stay there long periods at a time.
That ape head trophy displayed in Achenar's room on the Mechanical Age? Just some animal Achenar hunted once? Well, the inhabitants of the Channelwood Age were said be "monkey-like" in the journal, and Achenar had that chop-you-in-half table.
The Wahrks are essentially big fish that eat people, right? Actually, at one point in the Survey Island observation room, you can see a wahrk blow bubbles out of its blowhole (I think it's the first time you call the wahrk). So they must be mammals. Big mammals ... that are also notorious human predators. Suddenly Gehn's misspelling makes a lot more sense: WHARK = WH Ale + sHARK.
The advanced steampunk mechanisms inspire a lot of this in any Myst game, but some of them are downright odd:
To get to the lakeside pier from Yeesha's room, a combination lock must be solved - but a simple lever opens the door from the other side. It's like having a backdoor to your house that is only locked against the inside.
That is repeated, in a much more understated fashion, by the door between Catherine's study and the Myst III sun room. That might be a bit less justifiable than Yeesha's example (young girl more concerned with having a small private spot rather than security risks), however, given that the sunroom doubles as the designated linking chamber for anyone in a different Age who is able to link to Tomahna.
The kitchen and living room is connected by a single rotating bridge to either Yeesha's or Atrus' room. That has to be harder to build than just making two bridges, and also less convenient to use.
This could be a nod to Atrus's great desire for security. Having such a design allows for cutting off the kitchen and Yeesha's room from the rest of the house like a "safe room".
Also, if someone is in the position to use Yeesha's pier as a "back door", then they've either swum or climbed up the waterfall, or climbed over the cliffs surrounding Tomahna. In either case, there are better places to gain access (the former; the Sirrus/Achenar link chamber, the latter; any of the roofs.)
Sirrus has spent twenty years on Spire, having brought nothing with him but (if we're generous) a backpack of camping supplies and maybe a screwdriver. He built, from scratch, an entire high-voltage power plant and routing system. Okay. And he built needlessly intricate door mechanisms and locks in an Age that had no other inhabitants. Okay. But machine-tooled steel grate walkways and floors?
Perhaps some or all of the amenities (such as they are) in either world were written in during the original creation of the worlds. Atrus wanted to create prisons, not deathtraps, so including stuff like working infrastructre ( on Spire) or a bizarre broken cargo ship (in Haven) would be a way to help the imprisoned being survive.
More Bamboo Technology than steampunk, but Achenar's security measures seem rather wonky also. He supposedly built one of the gates to keep the local scavenger-critters from swiping his stuff, yet didn't the main thieving species in Haven have wings? How's a gate supposed to stop flying creatures?
I had been playing URU for a long time with different characters before I got to the online version. One Relto page later, I stopped, looked at what the page had added, and had to leave the computer for a few moments when I found out that Relto was a copy of Myst. Some of the things Yeesha had written made a lot more sense after that.
It was the mountain page, right?
For me it was the clock page that made me realize. I didn't catch on for quite a while.
The jetty page did it for me.
The Uru box cover does shout this fact out. Relto is shown at the same angle as Myst Island on the original Myst cover.
It was fifty-odd years between the writing of the Book of Earth and the D'ni's arrival in the Cavern. Well, we know from Esher that the Great Shaft alone is almost five kilometers deep, and if the map in Book of Ti'ana is to scale, then the Cavern is twice that. You'd need to install countless huge fans to cool things down enough for habitability.
Bittersweet Ending: End of Ages is mostly happy, with The Bahro freed and Atrus' family once again reunited. The Fridge Horror of this ending only sets in with URU Live, when the ideological schism of the Bahro causes a civil war and the death of at least one innocent person.