What is Diamond triangle of Buddhism?

In Orissa three hills Lalitagiri, Ratnagiri and Udayagiri and their environs comprise a remarkable Buddhist complex. Hiuen T’sang. The Chinese Pilgrim found it to be the seat of a flourishing Buddhist University called Puspagiri. Extensive ruins of brick pagodas, sculptured stone portals and esoteric Buddhist images testifying is ancient glory, have been unearthed. Ratnagiri is the gem of this complex. The magnificently carved door jamb of the Vihar and the superbly finished Buddha images form perhaps the greatest concentration o the post-Gupta period Buddhist sculptures.

Lalitagiri a holy relic of the Buddha was unearthed in 1985 by Archeological Survey of India. Excavations at Udayagiri, popularly known as Sunrise Hill, had brought to light the remains of a huge monastic complex comprising a Mahastupa and an east facing square monastery. The monastery at Udayagiri had a shrine chamber with an ornate gateway housing a colossal image of the Buddha.