Earth inner core is liquid whereas its outer core is solid?

I see a question mark but there is no question.

You should remember that nobody has gone more than a couple thousand feet underground. A Russian team drilled about seven miles, and oil explorers have used sound waves to compute what's within a few thousand feet. We have no knowledge of what is below that. Only guesses. The only fact we know is that our planet is hotter inside than outside. And we don't know if that is true everywhere.