Borobudur
On Java, the Sailendra kings built the largest Buddhist monument on earth — two million blocks of stone with no mortar, arranged as a diagram you walk: three miles of carved galleries spiralling upward through scenes of desire, then discipline, then emptiness, ending in bare stupas at the top. The building is the teaching. Within roughly a century it was abandoned, as Java's power moved east and the island later turned to Islam, and it sat under volcanic ash and jungle for something like eight hundred years.
A belief system rises, spreads, institutionalizes, and then schisms — changing the rules people accept as legitimate.