Mythical Mayan Underground Found (or so they think)

Archaeologists again think they have found the mythical underground passage to Xibalba, the Mayan equivalent of a legendary underground city, imhabited by superheroes and Gods. Xibalba is supposedly the place they vanished into when their civilization came to an abrupt end around the Middle Ages.
Let's see. What they found is a bunch of caves, submerged or not, some temples, some offerings, some bones. And of course the find is not the only one of it's kind, since the Yucatan peninsula is filled with enormous cave systems and Mayan remnants of worship.
The Xibalba myth is similar to the Acheron ancient Greek Myth, the Greek Underworld where Hades, the god of the dead lived along with heroes and demigods. But the myths have similarities of passage as well, complete with paranormal guard-dog, Indiana Jones - type obstacles and more.
The question here is this: Why did those people locate their mythical entrances on sites so easily explored, and clearly not leading anywhere? It's not that the ancients where not good explorers or easily intimidated by hardship, as proven by their presence in the remotest and most desolate of locations.
Was it that they dared not venture in a place clearly marked as sacred? Did they blindly believe their own myths? Or was it something else entirely?
Could it be that the caves where "open" only under certain circumstances, or on specific dates?
If such a thing as a portal to somewhere else exists, and is influenced - as ancient traditions state - by earth currents and proximity to water, would it be eternal?
The portals (known as Einstein-Rosen bridges in physics) if they where ever there, seem to have been long gone from these places, leaving only the meagre offerings of the people. And if they are gone from there, they must be relocated somewhere else, mustn't they?
How close to home could they be? And would we know?