“It is recorded in one of the Vinaya Sutras, that the historical Buddha Shakyamuni said that future Buddhists, who were not able to meet with him, should gather in one of the four great places of pilgrimage. These places are Lumbini, where the Buddha was born, Bodhgaya, where he attained enlightenment, Sarnath, where he taught for the first time, and Kushinagar, where he passed into Parinirvana. From among the four great places of pilgrimage, Bodhgaya seems to be the greatest, because here the historical Buddha Shakyamuni actualized the result of all efforts he had made throughout three countless aeons.
This is the place where all Buddhas of the past have attained enlightenment and where all Buddhas of the future will attain enlightenment. At the very top of the great stupa of Bodhgaya there is a smaller stupa, built by Nagarjuna. This stupa contains innumerable relics of the historical Buddha Shakyamuni. The Buddha statue in the shrine-room at the bottom level of the stupa was crafted before the time of the Indian buddhist king Ashoka, that is, approximately a hundred years after Buddha Shakyamuni entered parinirvana.”

(Explanation from Khenpo Tschodrak Thenpel Rinpoche, at the occasion of the first Kagyu Monlam with the 17th Karmapa, in 1996. Source: source: http://www.karmapa.org)