Generating new caves in old worlds

In a old world caves should generate in chunks you’ve been to the surface of but not the underground. If you have a old world that you’ve played in a lot you’ve probably explored thousands of blocks. And in that world you probably have lots of resources and things that make you want to stay in that world. But if you’d want to explore the new cave generation you’d have to go extremely far to find new caves. So I think that the underground should generate separately from the overground. I don’t know how to code games so I don’t know if this is possible/extremely time consuming but if it isn’t it would be a great thing to add.