Take a look at this pic
http://fantasymaps.files.wordpress.com/2008/06/hollow_earth.jpgLike my lovely wife said, it shows a map of advanced civilizations withing the earths crust.
I suppose in theory that may be possible...seeing as how we have yet to have the ability to drill far enough into the earth to see how far it actually goes (also the possibility of spewing magma all over the freaking place if they drill too far...always a great time)
Anyway, even if this was possible...take a look at the dead center of the map...
Central sun...
Now...if there were a sun (or something even relatively as hot) within the planet, wouldn't that just send the whole place up in flames.
Everyone knows that if you get to close to the sun you're going up in a fiery ball of doom...so...
How exactly is there going to be one -WITHIN- the planet...
I don't know...I just don't see that being possible...or ending well
Just my thoughts though...I'm not too book smart like a few others here...but still
Something about that just sounds off.