Information about the activity of the Atlanteans in Tibet and what they left for their descendants can be found in the book  by Lobsang Rampa.
How degradation of the Atlantean culture in Tibet happened — this we do not know. What we do know is that Tian tribes invaded Tibet… By the beginning of the second millennium the religion of Tibetans (Bon) did not prevent them from launching predatory campaigns against their neighbors, killing one another in the struggle for power, sacrificing animals (including horses, to which they would break their legs first) to “malicious gods” … 
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Buddhism came to Tibet from India in the VII — VIII centuries A.D. and gradually, though not without difficulties, started to prevail  in this region. What can be said about its Tibetan form that exists today?
On the one hand, Tibetan spiritual adepts of this tradition developed very efficient systems of training of the consciousness that lead to the highest spiritual achievements [21,27,39,62].
On the other hand — the principle of Love-compassion, which the Founder of Buddhism — Gautama Buddha — made the essence of His Teachings, was abandoned in the mass Buddhism of Tibet. Namely, the killing of animals was permitted. Moreover, black magic was “legitimized” in the form of seeking cooperation from non-incarnate hellish beings that started to be named “wrathful deities” .
A reasonable person understands that “whomever one is friends with, from them one learns”. Wise people seek association with the Primordial Consciousness. But foolish ones… What happens to them is described in the book , where I gave an example of followers of this perverted tendency in Saint Petersburg.
Those who were supposed to explain all this to the believers were Dalai Lamas. But…
Moreover, it is in Tibet that the concept originated stating that Perfection can be attained not by means of cognizing the Primordial Consciousness and Mergence with It but as a result of “momentary enlightenment”. It was believed that for the sake of attainment of this “momentary enlightenment” disciples had to be insulted, humiliated, and beaten, preferably when they do not expect it.
The following story was included in the “heroic epos” of Tibetan Buddhism. A master with his disciple who were starving finally approached a village. The master sent the disciple to ask the villagers for some food. The latter came back with the food that he had been given, hoping to also satisfy his hunger. But the master devoured all the food himself… and told the disciple to drink a stinking slush from the runoff ditch… And then beat him with his dirty shoe on his face… The disciple, as the story goes further, attained a “momentary enlightenment” as a result…
(This tendency of obtruding this kind of “enlightenment” on other people became popular among many pseudo-Buddhists with devilish souls, especially those of some extremely perverted “Buddhist schools” in China and in other countries, including Russia).
To make it easier for the readers to understand what it is all about I should clarify that the term Enlightenment implies transformation of the individual consciousness of embodied man into Light, when it gets close to the state of the Creator. (Remember: “God is Light!...” (1 John 1:5)). This Light-Love then starts shining out of the adept’s body. Man acquires such a state, which we have already discussed, as a result of the refinement of the consciousness and growing it in the subtlest eons. I must emphasize: this can be achieved only through the growth of consciousness as refined and tender love-peace, followed by Mergence of it with the Fiery Manifestation of the Creator (or “Clear Light”, in Buddhist terms). True Enlightenment can be attained only as a result of realization of the functions of the spiritual heart. And not by means of cultivating aggressive arrogance, rudeness, and the ability to suppress other people with the power of the devilish-quality consciousness.
But modern proponents of Tibetan Buddhism coming to Russia teach their followers not development of the spiritual heart and refinement of consciousness but… to plug the lower foramens of the body with corks — so that one does not fall down to hell out of them…
…Thus, in Tibet there was the same typical situation of gradual degradation (after a period of development and culmination) of great and pure Teachings which were displaced by human ignorance.
… And the Chinese Cultural Revolution swept Tibet away.