I am not convinced by that idea since there are many missing items to really backed up that claim. But I do not set aside the possibility that there is a civilization that is very advanced that is living within the planet. The hollow earth theory according to the believers would explain why the tibetan monks have some profound wisdom on reality since the elite beings on hollow earth inspires them. There are also paintings of flying objects in Indias history which they say are gods and probably those gods were the advanced civilization living within the hollow earth. It is hard to believe but I am looking at that possibility as a potential truth and thus I am still continue to read some article that probably explains about hollow earth. But as of today I do not say it is true but only a potential probability.