Yes, yes, I saw the Templar cave story.
The Templar Shropshire Trap
The short version about that supposed Templar cavern found in Shropshire? The true, hidden version? It was never dug out, or even adapted, by the Templars. It has nothing to do with the Templars at all. The Templar cavern was instead carefully constructed in 1965 AD or so by an esoteric conspiracy with no love for humanity, and an obsessive desire to transform the world into something more to that conspiracy’s liking.
The background comes right out of GURPS Horror: The Madness Dossier: this reality that we live in right now (‘History A’) was created in 535 AD as the culmination of a rebellion of humans against their alien Anunnakku masters. The previous reality (‘History B’) was buried in a semantic and metaphysical ‘reality quake’ that wiped out an entire world, and put a new one in its place. But there were refugees from History B (‘irruptors’); and they struggle even today to put back the old world and bury our own. And, again: they have no love for humanity.