Endless Sky?

So I've yet to see a starbound video where when they go deep and dig out the background there is some rock background that cannot be touched that indicates you are literally underground. I've seen many videos where when they go deep and dig out the background there is sky revealed and this makes me think... you're on a mountain? Has anyone seen evidence of a untouchable underground background?

 

Yes, On the 7 part stream you see a few rock biomes in the background. Tiy even points them out.

 

I know what you're referring too, and yes, they were on a mountain. As the post above me kindly put it, there are underground backgrounds. If you go far up enough you'll eventually exit the atmosphere, and if the planet have rings, you'll probably find some asteroids there.

Unless you actually mean that the "block background" turns invincible further down, in which case, no, there's been no hints that that's the case.