Doing some logical thinking about this now..............
They could be lava tubes. Since they are apparently centuries old, I would assume this is the most logical choice.
Nature has always created its own fascinating "work arounds" at times. Animals have better senses than humans and so do insects, and this could be why they didn't want to near the hole, and also why plant life thrived around these holes.
The animals and insects noted the scent of volcanic residue, which would also indicate why plants loved it....as volcanic residue is like a free buffet for plant life, lots of minerals to live on.
As for all this other stuff........I doubt any of it is true, except for the part about the military showing up. They usually do in instances like this.......not just to see exactly what is going on, but to see if it's dangerous enough to confiscate and quarranitine.
If they don't find anything noteworthy there, they will just cover it all up and leave. If there is something worth their time, then they will usually section off the area and build some sort of station there.
In any case, it's been "covered up", so we will never really know about the hole itself.
As for all the other parts of his story, I expect its all baloney. If these Basque really DID photograph and record everything they did, then WHERE is it all? After all these years, something would have surfaced online...........recordings, videos, photos........SOMETHING.
So, I think this is more of a case of somebody wanting to "go down in history" by concocting a story around one of these holes.
