Bagan holds more than two thousand surviving temples out of perhaps ten thousand built, and it spent decades excluded from the World Heritage list for what was done to them in the name of repair.
The building period
Bagan was the capital of the first kingdom to unite the region. Construction peaked between 1150 and 1200. The kingdom fell in the aftermath of the Mongol invasions of 1287, though Mongol armies may never have reached the city itself.
The religion of the building period was not simply the Theravada practised there now. Mahayana and tantric Buddhism, Hindu deities and local nat worship were all present simultaneously, and the iconography still shows it. The later Theravada consolidation is a reform imposed on a plural inheritance, not a continuous state.
The earthquakes
The plain sits in an active zone. A major earthquake in 1975 damaged a great many structures, and another in August 2016 harmed roughly four hundred.
This is the fundamental condition of the site: it will be shaken again, and any conservation policy is really a policy about what to do afterwards.
The reconstructions
The answer given by the military government through the 1990s was to rebuild, fast and at scale.
The work used modern materials, concrete, unmatched brick, on medieval fabric. It invented architectural forms that had never existed, raising complete superstructures on foundations that had only ever been foundations. Original brickwork was covered or replaced. A golf course and a viewing tower were added inside the zone.
The objection was not aesthetic. Reconstruction of that kind destroys the evidence: once a hypothetical superstructure is standing in new brick, the question of what was actually there can no longer be asked of the building.
The cost
UNESCO withheld inscription for decades on exactly these grounds. Myanmar had nominated the site as early as the 1990s.
Inscription came only in July 2019, after commitments on conservation practice, the removal of some intrusive structures and a change of approach.
Set beside Prambanan, where the rule is that nothing is rebuilt without its own original stones and dozens of shrines are consequently left as bare platforms, Bagan is the standing argument for that restraint.
Still in use
None of this is archaeology in the sense that Ajanta is. The larger temples are in daily Theravada use, with resident monks, gilded images and offerings, and the surrounding villages have never stopped using them.