The biggest cover-up of all time is the fact that there is a civilization of people living in the center of Earth, whose civilization's name is known as "Agartha".This may be hard for some of you to believe. I know it was for me at first, however, I now have an absolute knowingness of the truth of this.
The Indian epic, the Ramayana and the Bhagavad Gita are the two most famous texts of India. The Ramayana tells the story of the great Avatar Rama. The Bhagavad Gita tells the story of Krishna. The Ramayana describes Rama as an "emissary from Agartha", who arrived on an air vehicle. This is quite extraordinary in that both the Buddhist and Hindu religions separately refer to Agartha.

The Buddhists, in their theology fervently believe in its existence. They believe it to be a race of super men and women who occasionally come to the surface to oversee the development of the human race. They also believe that this subterranean world has millions of inhabitants and many cities, and their capital is Shamballa. The Master of this world was believed to have given orders to the Dalai Lama of Tibet, who was his terrestrial representative. His messages were being transmitted through certain secret tunnels connecting this inner world with Tibet.
The famous Russian channel, Nicholas Roerich, who was a channel for the Ascended Master, El Morya, claimed that Lhasa, the capital of Tibet, was connected by a tunnel with the inner earth, Shamballa. The entrance of this tunnel was guarded by lamas who were sworn to secrecy. A similar tunnel was believed to connect the secret chambers at the base of the great pyramid at Giza, and Agartha.

The first public scientific evidence occurred in 1947 when Rear Admiral Richard E. Byrd of the United States Navy flew directly to the North Pole and instead of going over the pole, actually entered the Inner Earth.

http://humansarefree.com/…/hollow-earth-agartha-complete.ht…

There are Puranic evidences that are said to point to a civilization inside the hollow earth. Some of them include:

1. Sons of Sagara and the Patala Ganga (Brahmanda Purana)

Indra had stolen the sacrificial horse of King Sagara which he had kept for the ashvamedha yajna (sacrifice ritual using a horse). In their search for the horse, the sons of Sagara came to a Northern ocean, which they crossed, and entered the interior of the earth. There they found the horse in the hermitage of Kapila Rishi. They mistook that it was the Rishi who stole the horse and disturbed his tapas (meditation). The angered Rishi burnt all the 60,000 of them to ashes with a curse. The horse was later found by the grandson of King Sagara and the asvamedha yajna was completed. Finally, it was Bhagiratha, a descendant of the late King Sagara who performed intense tapas and brought the Ganga River to the world and then to the Palata Loka where she washed the sins of the sons and liberated them.

2. Kalki Avatar of Lord Vishnu

Srimad Bhagavtam states in Canto 12, chapter 2, Text 18:

sambhala-grama-mukhyasya
brahmanasya mahatmanah
bhavane visnuyasasah
kalkih pradurbhavisyati

According to this verse, Lord Vishnu will take the Kalki Avatar at the Sambhala village at the end of Kali Yuga to establish the oncoming Satya Yuga. Padma Purana (6.242.8-12) also mentions that the name of the village is Sambhala. This village is supposed not to exist today, but to be be born in due time, from the inner earth. The Tibetan lore speaks of a city named Shamballa which they consider to be in the interior of the earth on the Himalayas. Thus the Satya Yuga with its truly Vedic civilization will be established on the surface of the earth from its interior.