China is on the opposite side of the world from America. Therefore, it is the hardest country to "dig to", which is where the expression came from. It has many uses. For example, if children are digging in a sandbox a parent might say "Are you digging a hole to China?" Meaning the hole is very deep or the children are putting a lot of effort into digging. Also, the task is obviously impossible so it could be used in that way. I can't give you any other examples because, honestly, I haven't heard that expression in a long time. But I hope I helped you anyway :)

@祖父：Haha. You can swim across it. :) @GLYN: I see. That's because the opposite side of the world from our own country is different. Then in China should we say dig a hole to America? Haha :)

I England we say "Are you digging a hole to Australia"

ironically, the opposite side of the world from me is actually just off the coast of america ... :)

@dancingbear I heard it from a TV series. Thanks for telling me that. =)