The Jetavanarama Dagoba, which was the pet project of King Mahasena (274-301AD), is the largest building at Anuradhapura. Originally it was no less than 122 metres high, making it the third tallest structure on earth after the pyramids at Dharshur and Gizeh in Egypt. The Jetavana Dagoba must have dominated the ancient city, just as it dominates the ruins today, even though its height has diminished to about 75 metres. But whereas for centuries its dome has been encrusted with vegetation and its spire broken, it is now being restored with support from UNESCO.

Mahasena became a convert to Mahayana Buddhism under his tutor, and when he came to the throne he persecuted the sects that opposed the new doctrine. Hundreds of their buildings were razed to the ground, including the Brazen Palace, and the materials were used for the erection of shrines and monasteries for the new sect. However, after a lapse of some years, Theravada Buddhism still held sway and Mahasena's monarchy became endangered, so he returned to the old doctrine and restored all the buildings he had destroyed.

The Jetavanarama Dagoba dates from the middle of Mahasena's reign, before he had recanted, for the Mahawamsa makes no bones about the fact that it was built for the Mahayanist Abhayagiri monastery: "The king, having had two brazen images or statues cast, placed them in the hall of the great bo-tree; and in spite of remonstrance, in his infatuated partiality for the thera Tissa of the Abhayagiri, fraternity - a hypocrite, a dissembler, a companion of sinners, and a vulgar man - constructed the Jetavanarama for him, within the consecrated grounds of the garden called Joti."

The massive weight of the Jetavanarama Dagoba required the construction of a circular platform (salapatala maluwa) 113 metres in diameter. Archaeological investigations have revealed that the platform rests upon a brick foundation 10 metres deep, which in turn rests on a substantial layer of concrete-like material. The dagoba itself was built of large bricks laid in a mortar of adhesive clay. The brickwork was then covered with a coating of lime plaster and painted white.

Such was the amount of material required that James Emerson Tennent calculated in his book Ceylon (1859): "The contents of the semi-circular dome of brickwork and the platform of stone seven hundred and twenty feet square and fifteen feet high exceed twenty million cubic feet. Even with the facilities which modern invention supplies for economising labour, the construction of such a mass would at present occupy five hundred bricklayers from six to seven years. The materials are sufficient to raise 8,000 houses, each with a twenty foot frontage, and these would form thirty streets half a mile in length. They would construct a town of the size of Ipswich or Coventry; they would line an ordinary railway tunnel twenty miles long, or form a wall one foot in thickness and ten feet in height, reaching from London to Edinburgh."

With the ultimate collapse of the Anuradhapura civilization the jungle tide swept in, and the dagoba took on the appearance of a wooded hill. Beneath this vegetation, however, lay brick not earth, which prematurely disintegrated under the assault of the roots. As one 19th century visitor wrote: "Even to the highest pinnacle it is encompassed and overspread by trees and brushwood; these are the most active agents of ruin to the ancient buildings of Ceylon, as their increasing roots and towering stems, shaken by the wind, overturn and displace what has long resisted, and would have slowly yielded before time and the elements."

Excavations conducted under the Cultural Triangle Project since 1981 have uncovered a considerable collection of artefacts, such as intaglio seals made from semi-precious stones and glass portraying human heads, animals and birds. In addition, Roman, Indian and other foreign coins have been found, together with fragments of glass from containers offered in enshrinements, beads semi-precious stones, crystal agate, ivory, bronze, gold and silver. Next to the dagoba stands the Jetavanarama Museum, which houses some of these finds.