[Squirrelloid]

the name Agartha (probably Agartta actually, its the more common spelling) may be indian, but the concept is very much western European/American at the point it becomes mythologically significant, at least as far as I'm aware.

So, going back to ~17th century, this idea that the world was hollow, and there was something inside you can visit, started to become popular among certain alchemists and other occult thinkers - and these were often men of science and learning as well. The idea may preceed Hailey's description of the earth's magnetic field as being plausibly caused by the earth being a series of shells one inside the other that rotate somewhat independently, but certainly when misinterpreted that statement creates the idea of independent worlds nested within one another. (Its actually a pretty accurate description when not misinterpreted).

Fast forward to the 19th century and there were a large number of pseudo-intellectuals in the US (and probably Europe) who believed in a Hollow World. To some of them, Agartta was the civilization who lived on the inside (and was generally endowed with mystical powers or advanced science - any sufficiently advanced science is indistinguishable from magic after all). Now, how you *got* to the inner world is where the association with teh Himalayas comes in. I don't know if Agartta was specifically used by those people who believed an entrance existed in the Himalayas or not, but certainly the Himalayas and a few choice other locations (both poles) were given by various adherents as the entrance to the inner world. Basically, anywhere sufficiently inaccessible that it couldn't be disproven (at that point).

So that's where Agartha comes from.