Well, as even descending four kilometres under the surface you would be facing such heat that wouldn´t be pleasant (such as 60 degrees celsium), descending 700 km would be.. do the math. Do you like the word "death-bringing"?
That as well governs the question about life. It is better to think about it as a tighly packed dirt (well, presured ringwoodite/hydrous minerals) full of water, that is not evaporating because of the pressure.
Interesting, but I am afraid you should not take Sanji in there.