"Phra That Phanom is located within the walled compound of the "Temple of Phra That Phanom" in Nakorn Phanom Province, Thailand, about 600 meters west of the Mekong River. Believed by the people in the region to have been built by Monk Maha Kassapa, a chief disciple of Buddha, to enshrine the Buddha's breastbone relics 8 years after Buddha's Nirvana, this Stupa is regarded as the most ancient Buddha's Relics Stupa in the Northeast of Thailand and in the Middle Mekong River Basin. Throughout the millennia, Phra That Phanom has been steadfastly venerated by the multi-ethnic and multi-national populations living in all directions of the region."
Source: UNESCO World Heritage Tentative List
I visited Wat Phra That Phanom, which is north of Mukdahan town, along the Mekong River opposite of Laos. Got the cancellation from the post office next to this famous Buddhist temple in October 2018.