Earth has radius of around 6378 km and our technology are limited to explore only few miles below earth's surface. In that case is there any unknown world exists hundreds of miles below earth surface like in hollywood movie "A Journey to the centre of earth"? Is there any civilization exists which can live at higher temperature found below earth's surface?

If I was to take a huge heat-proof tube and stick it through the very centre of the earth and then jumped down it, what would happen when I reached the middle? Would I float at the very centre?