I've seen the following two seeds generate infinite caves:


But why does this happen?

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    Not sure why people bothered putting this on hold since it's already been answered and accepted – Phoenix Aug 5 '17 at 20:11
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    @Phoenix whether or not a question has been answered sufficiently has no bearing on its topicality. – Vemonus Aug 5 '17 at 20:35
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    I just don't see what holding it changes - the process has completed, there's nothing to hold – Phoenix Aug 5 '17 at 20:37
  • @phoenix There is a very good reason to do so; our site is meant for long term. Not just helping the immediate user, but to build a repository of knowledge that withstands the test of time. Closing a question that doesn't fit is meant to show that it is problematic. That it has been answered, or can be answered, isn't part of the evaluation process, and actually misleads from the problem. If the question is bad, or doesn't fit, closing it shows that we don't want these types of questions. It also allows is to delete it quicker, thereby ensuring our site is better organized. – Frank Aug 5 '17 at 22:33

This is from this page:

Seed 107038380838084 breaks Minecraft in interesting ways. It generates infinite mineshafts, caves etc. This happens because Java's Random() function returns zero with this seed and then Minecraft breaks in interesting ways.

Some more detail from this page:

Note that only caves and mineshafts are affected by the repetition bug since it is a result of a multiplier value being set to 0 and they are the only ones that use it, at least directly (the game derives a unique seed for each chunk by multiplying the x and z coordinates by two numbers derived from the world seed, and either (but not both) can be 0, resulting in the corresponding coordinate having no effect on the chunk seed. Villages and temples use an additional algorithm, which is immune to this bug, to determine where they are located within a given 32x32 chunk region but should still generate the same if they have the same x and z offsets. Features like trees and ores are not affected at all because the multipliers are made to always be odd numbers, which implicitly excludes 0).

More from the same page, something I thought was interesting:

In addition, as mentioned in the second thread above virtually every seed has a bug (due to the way the chunk seed is determined) that causes the same caves and mineshafts to generate at sign-reversed coordinate pairs (e.g. 100, -100 and -100, 100), with as many as 1/3 of all chunks matching (when I investigated this further a few seeds seem to be immune and the percentage of chunks that matched was always 1/(3*n); e.g. 1/3, 1/6, 1/12, etc).

Sounds like 0 is used as a multiplier for the 'random' world generation and so the same section is repeatedly generated. Also, those are not the only two seeds. There are tens of thousands if not more.