“Phra Pathommachedi or Phra Pathom Chedi (Thai: พระปฐมเจดีย์) is a stupa in Thailand. At 120.5 metres (395 ft) it is the tallest stupa in the world

The name Phra Pathommachedi means the first holy stupa, given by Mongkut. Originally the stupa named Phra Thom Chedi means the big stupa in ancient Khmer language or the royal stupa in Northern Thai language. One of the common misunderstandings about the stupa is Phra Pathommachedi is the oldest and the first stupa in Suvarnabhumi, an ancient name of Southeast Asia. Modern Historians believe that the stupa was one of the principal stupas of ancient Nakhon Pathom, the largest settlement of Dvaravati culture in Nakhon Pathom area together with the nearby Phra Pathonnachedi (Thai: พระประโทณเจดีย์) during the 6th to the 8th centuries.” (1)

[1]-http://en.wikipedia.org/wiki/Phra_Pathommachedi