A couple of weeks ago, I created a randomly-generated world that turned out to have a nice little island. I tried it out for a bit and liked it, and want to re-create it with some of the extras from some mods I have gotten since then. The problem is that I cannot seem to regenerate that same world anymore. Using the same seed creates a variety of worlds that range from almost the same (a more-or-less similarly-shaped island) to vastly different (a field of flat ice as far as the extreme-eye can see).

I tried regenerating it with vanilla, but that creates a tiny island in the middle of the ocean (which is amusing, but not what I want). Obviously some mod or other is responsible for creating the original island and some sort of interaction with other mods is responsible for messing it up.


The problem is that with a few dozen mods, testing each and every permutation would take at least two or three eternities (especially since start-up and world-generation get slower and slower with more than a few mods).


A more practical solution is to narrow down the list of mods to test by determining which mods affect world-generation and test different combinations of just those.

The problem now is: how can the abilities/effects of mods be determined? That is, how can a group of mods be examined to determine which ones affect world-generation, or add mobs, or tweak enchantments, and so on?

Obviously, checking the web-pages of each mod is impractical and unlikely to return all the relevant information anyway.

(I’m not looking for specific information about which blocks or algorithms or what-have-you a mod creates or affects, just whether it does or not. A typical result might be a table of effects that mods can have along the top and a list of installed mods down the side with checks in the appropriate boxes in their rows.)


Is there a way to examine the mods directly (ideally with a tool rather than by hand) to find out what they affect by examining the code? For example, can mods that tweak terrain-generation be determined by checking for overrides of some specific .class file or function?

  • For those that find themselves in the same situation as myself, it’s possible to to fix the issue without the tool I’m hoping to find (though I’d still love to find a solution to the question above). To fix this specific problem of regenerating a world affected by mods, open the level.dat file from original world in NBTEdit, expand the FML→ModList branch, then use SysExporter to grab a text dump of the mods that were active when that world was created.
    – Synetech
    Nov 28, 2015 at 19:47
  • Just suggesting you can generate the island you like first, with whatever mods you had, then turn on retro-gen in the configs of the new mods to generate new features on existing terrain.
    – Moddl
    Nov 29, 2015 at 3:31

4 Answers 4


Using a tool that is able to search inside of zip files, you could search your mod folder for worldgen-related terms (eg. biome/generator/...). This is not certain to find every relevant mod, although it might help you to narrow it down.

If you are really interested in the world, you could decompile the source files and then search them for worldgen event registrations: every mod that tries to modify world generation has to 'sign up' to get involved in that process, and this 'signing up' call should be very easy to detect (you can look up the specific event names in the minecraft forge forums).


If the mod is open source, you can look at its source code.

Most popular mods are documented in wikis, like here: Ex Nihilo, Draconic Evolution, Botania, Mystical Agriculture, Optifine, ... But many smaller or less popular mods are sadly very badly documented.

The Minecraft forum post or whatever website it's mainly published from usually has a long explanation from the publisher themselves.

If all else fails, it's possible to decompile Java files and look at a variant of their source code that way. If you want to do that, it's best to search for a tutorial.


-how can a group of mods be examined to determine [their impact on the game]?

The most specific answer I can give is to get a group of mods together and test them, though that isn't very helpful. I see your issue; most mods don't change Minecraft in one specific way. I'm sorry but the best advise I can give is to remove mods in large amounts until you find what you're looking for, and then remove mods from that shortlist through the same method.

Alternatively, you can copy the dat file named "level" and overwrite a vanilla worldsave with it (but if you've done any mining, you'll be missing those things), and then after loading it in vanilla, send it back over to your modded version.

You could also try McEdit to copy the island and paste it into a different world (or save it as a schematic). If you want to generate the exact same island, untouched by Steve hands, I'm sorry I couldn't be more helpful, you'd might just have to ssettle for something very similar. I wish you the best of luck!

P.S: Don't forget to back-up your [everything]


You can go to the .minecraft folders then click on the saves folder. After that find the name of your world then duplicate it. When you run minecraft you can scroll to the bottom and you'll find your world. In this world everything is going to be the same.

  • Yes, but OP specifically doesn't want the same world. He wants the same terrain, but including features of some mods, but one of them seems to modify world generation.
    – MrLemon
    Feb 5, 2016 at 11:21
  • Oh okay sorry I don't know. Maybe mcedit could help??
    – MrPungPond
    Feb 8, 2016 at 9:45

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