The study of abandoned civilizations, and of the reasons for the disaster, is a field in which many imaginations have wandered. In several parts of Southeast Asia, notably in Cambodia, northern Siam (the Kingdom of Haripunjaya), the Pagan area in the dry zone of Burma, and the dry zone of Ceylon, this pattern has been repeated. Only in Ceylon is there a more or less continuous historical record, plus numerous stone inscriptions, to assist in solving the puzzle. Otherwise the cases are suggestively alike. All arose on reasonably level plains (often the only level land in the region), in a climate of alternating wet and long dry seasons, which especially in the tropics usually means irrigation if agriculture is to be productive enough to support a more than primitive civilization. All did in fact depend heavily on extensive irrigation, and appear to have shared the characteristics which Wittfogel has associated with this kind of basis: a strong central state, massive public works, a highly structured society, and a powerful ramified bureaucracy. All were plagued with chronic invasions from nearby densely populated areas, and all collapsed with dramatic suddenness, to be blotted out by jungle so that with some even their memory was forgotten. Finally, all were abandoned at about the same period, the thirteenth century (Angkor Vat somewhat later), and with few exceptions no significant attempts were made to reoccupy them until very recent years.