Agartha

The kingdom inside, capital Shambhala, lit by an inner sun. Expeditions keep not returning.

The kingdom under the surface arrives independently in Tibetan texts (Shambhala), Hindu tradition (Patala), Hopi emergence stories, and nineteenth-century French occultism, which named it Agartha and gave it a king — the one the Ahnenerbe went looking for. The consistent details across traditions: an inner light, a patient civilization, and tunnels that surface in the Himalayas, the Andes, and — per the modern files — under a mesa in New Mexico.