Almost every ancient culture has a story or allusion to the inner realms of Earth, as well as civilizations or people in the center of the Earth. Many of these are places of purgatory where dead spirits reside, rather than a society currently flourishing. The Greeks spoke of Hades, the Christians refer to it as Hell, and in Judaism it is referred to as Sheol. But most of these underground domains were more allegorical and religious tools used to deter people from sinning rather than realms actively being sought out.
There are, however, closer depictions of Agartha described by some cultures with related cities and passageways to get there. In Tibetan Buddhism, there is the secret, mystical city of Shambhala located somewhere deep in the Himalayas that many have searched for, including Russian mystic Nicholas Roerich, though no one is known to have found it. Some believe Shambhala could potentially be connected to Agartha.
In Hindu and Celtic lore — which some believe shared an ancient connection through a lost antediluvian city — there are caves and underground entrances to sub-terrestrial worlds. Some have connected the Hindu land of Āryāvarta, or “abode of the excellent ones,” a land ruled by a supernal race thousands of years before the great war laid out in the Mahabharata. Many believe this ancient race to be of the same lineage as the ancient civilizations from Atlantis, Lemuria, and Mu that were wiped out by war and cataclysmic events, driving them underground to Agartha.
There is another underworld in the Hindu Mahabharata known as Patala that others point to, as it shares many similarities with depictions of a subterranean world, though it is said they are at war with the Agarthans. Patala is the seventh layer of the underworld in Hindu scriptures and is ruled by the nagas, a half-human, half-reptilian species who are depicted with jeweled hoods that light their realm. The Naga are a highly advanced race with cutting-edge technology. Occasionally they are said to abduct, torture and kill humans, though other accounts refer to them as having a positive impact on Earthly events.
Will we ever find out if there exists an underground civilization below the surface of the Earth? There have been attempts to drill down as far as possible to see what’s there, and the Kola Superdeep Borehole made it about 7 miles. The Russian team that led the operation didn’t find Agartha, though it would likely be much further down.
What they did find showed we really don’t know exactly what’s down there. Many of our theories were proven wrong, and we learned a lot more about the Earth’s crust. For one, there was a layer of basalt, known as the Conrad Discontinuity, completely missing, while the presence of water was detected where it shouldn’t have been. This just goes to show: We don’t really know what’s down there.