From what I’ve noticed, the extractor actually digs out dirt.  So, it just assume you can have an endless hole.  I’m not sure if it has a limit, but it could make for an interesting way to dig out a 3x3 hole that can be filled with water.

so it's a little strange that the dirt extractors go forever,  thats not really how dirt works,  instead you could have a building that takes in food and water to make compost (dirt) it's functionally very similar but would require  more planing and would feel more inline with other "renewable" resources like trees.

thoughts on the endless dirt