I like the video of the inner world it reminds me of the lord of the rings. Most people that are into this sort of thing are well aware there are several ways into the inner earth including the Himalayas. There are entrances dotted around the earth. We have seen the Afghanistan caves and others in documentaries. We don’t know who built the tunnels but we know there are networks all over the world for a reason.
Technology that is thousands of years old must have been used sometime in the far distant past to make these tunnels. Modern man has only been able to do this in the past two hundred years with explosives and then the invention of boring machines.