So, mainstream scientists today believe that the Earth under our feet has a lot of molten rock and metal filling it and have gathered a lot of pretty solid evidence for it. The only complication is that we've never been able to send a human down more than several miles to actually study it up close, largely because No One Could Survive That. Which is why since times that are Older Than Radio, early scientists, writers and more than a few crackpots have believed that there just might be something...or indeed, someone (say, Ultraterrestrials)...down there, possibly powered by a suitably sized sun in the center.