Hidden Land. The most famous and enigmatic of these is the so-called Graydon Obelisk, though a similar crystal (an anonymous gift) resides in the collection of the Smithsonian's Department of Anthropology. Both these crystals and others rumored to exist figure prominently in Fortean and paranormal lore.
The account attributed to John Richmond Graydon (but only surfacing after his death) asserts that he found the crystal on a skeleton garbed as Spanish conquistador in the Sierra Madres. He describes the crystal psychically projecting voices and visions of another world into his mind. He came to understand the crystal was part of a control mechanism. In a trance-like state, while under the crystal's influence, he produced a crude map of the Hidden Land, the area maintained by this mechanism.
Graydon relates that those who have been to Hidden Land and returned confirm the existence of larger crystal obelisks: perhaps 16 ft. tall, and 4 ft. wide at the base in remote places in the Land. Sometimes they appeared are normal, cloudy quartz crystal, but at times colored pulses of light appear inside them. They provided some influence over weather and even astronomical phenomena--perhaps even time and distance. From his investigations, Graydon believed these to be part of system of smaller obelisks--most of which had been swallowed by the jungle.
The builders of these crystals and the grid they form are an enigma, but at least one of Graydon's correspondents attributed them to the Nephilim of Genesis. Graydon's account is conflicted in regard to whether these giants still exist.
Graydon was found dehydrated and dying from a spear wound in the Matto Grosso in 1908. His appearance (and eventual death) in Brazil presents something of a mystery as he dined with acquaintance in Tucson just two days earlier.
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