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[Foxite]

An extremely large DF world such as this one, when extracted from its zip file, weighs "only" 121 MB. I find it unlikely that the entire world, with all its glorious detail, fits in that space. Every single tile in every single layer, the position of every creature, every item, etc. I would think that the game only saves an abstract representation of what should be within given areas (such as the ores, the structures etc), and only generates the specific details when you go there in dwarf/adventure mode.

Of course, history would need to be saved, otherwise you'd be regenerating all of that every time you opened up legends mode. But I doubt that accounts for the majority of the 121 MB.

Does anyone know what the game does in this regard? I'm curious.

[Quietust]

The game typically just procedurally generates everything on demand - worldgen determines the elevation+precipitation+draininage+volcanism+salinity+etc. for the entire top-level map and uses that to assign biomes, plant/animal populations, and geology.

When you embark in a location (or explore it in Adventurer mode), it generates all of the terrain (and plants and animals) using that information and then saves it in full detail (eventually in a "site-X.dat" file once you retire/abandon or otherwise lose).

For Adventurer mode, it regenerates the terrain every time you visit it, but if you make any changes it'll keep track of them individually and then "re-apply" them when you visit it again in the future. This is why sleeping in Human towns causes you to forget where everything is - the terrain gets unloaded when you go to sleep and regenerated when you wake up. This is also why there are some bugs involving things changing when sleeping or revisiting areas - there are some things that are generated "randomly" (using "seed" values that are also saved within the world, so that it always generates the same random sequence each time), and sometimes there are things which don't re-seed the random number generator correctly (causing things to be different every time) or do so in the wrong order (causing things to differ based on the direction from which you approach them).

[Thisfox]

A good demonstration is with artifacts. If you get several artifacts in a row and don't go to check out what they are, then check them all later. Artifact A, made six months ago, might have an image on it of artifact B, made three months ago. It's like it was predicting the second artifact, but what actually has happened is that the details of the artifact were set at the time you looked, instead of the time the artifact was actually made.

[Shonai_Dweller]

A good demonstration is with artifacts. If you get several artifacts in a row and don't go to check out what they are, then check them all later. Artifact A, made six months ago, might have an image on it of artifact B, made three months ago. It's like it was predicting the second artifact, but what actually has happened is that the details of the artifact were set at the time you looked, instead of the time the artifact was actually made.

Although artifacts aren't such a good example as worldgen artifacts are actually generated in worldgen (since the artifacts release). Fortress ones do still seem to act as described though, yes.