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 = WHAle + sHARK.
If you start nit-picking the physics in the game, you'll probably notice anomalies such as shadows that don't cast consistently (Riven does not have the programming to emulate a day/night cycle, so this is odd) and an island with an elevator that takes you to two floors that are about the same heights above sea level.
Remember that in Atrus' diary (in your inventory from the beginning) that the world of Riven is accessible due to Gehn's careless writing style in the descriptive book. Think of a computer programmer who copies and pastes code to make a program, failing to realize that the code will probably interact in destructive ways, leading to a program crash.
Enforced by the designers. For example, it's entirely possible to use the wahrk gallows before figuring out its actual purpose. The smaller details of how Gehn views and treats the Rivenese, and how his rule has affected the Age's ecosystem, can also be easy to miss within gameplay.
If the player has read The Book of Atrus, Catherine's captivity takes on additional, deeply unpleasant layers of meaning.
Gehn has been trying to recreate the Linking Books and numbers each and every one of them, like what is ultimately the stable one he created, which implies he believes each world is its own Age that he made. He then burns those imperfect books. Put two and two together and it gives a horrifying implication about Gehn.
The notion that you're in a world hanging out by a thread, with Atrus being the only person keeping the age from collapsing into oblivion. The fact that lava is mere meters from the surface isn't very reassuring.
The swimming reptiles, beetles, cave frogs, wharks... all extinct by game's end.
Given that falling into the fissure does eventually safely lead the Stranger back to Earth/D'ni, Gehn could have actually freed himself, if he had been willing to jump into the fissure himself. Seems crazy he would even consider it, but reading his journal in his office shows he was at least wondering if it were feasible.
