This builds on a search that's been underway since the 1880s for the birthplace of the empire. That hunt was set off by a reference to a city in the most important of the inscriptions that the French used to reconstruct the lost history of the empire--the Sdok Kok Thom inscription, 340 lines carved into a monolith found at a temple that's now located just inside Thailand. (You can read a full account of this amazing testament in my 2010 book, Stories in Stone, which focuses on how the French recovered the lost history of the empire by learning to read inscriptions.)
Among other things, the Sdok Kok Thom inscription gives an account of the founding of the empire around the year 800 AD. The prince who would become Jayavarman II, first king of the empire, is described as moving from city to city, in the company of a priest named Shivakaivalya. "When His Majesty left Amarendrapura to reign on Mount Mahendra, the august Shivakaivalya likewise went and took up residence there, serving His Majesty as before." The text goes on to describe a Hindu rite carried out at this place to make Jayavarman king and the Khmers free of foreign domination. Historians generally treat this rite as the founding of the Khmer Empire.
The problem is that over time, place names fell out of use, making it a challenge today to identify exactly where particular events took place. It's always been assumed that this Mount Mahendra was located in the Kulen hills north of Angkor. But where in the Kulens? In the 1880s, the first translator of the inscription, the great Etienne Aymonier, climbed the Kulens looking for it, but did not succeed. Part of the problem was that Aymonier and others were convinced that there must be a fabulous "palace of Jayavarman" hiding in the forests up there. They reasoned that surely the empire's founder would have built something remarkable. For a while, attention focused on the huge Beng Mealea temple at the foot of the Kulens (in fact it was built more than three centuries later).
What's been increasingly documented in recent years fits more with what you'd expect for a temporary capital--a place of modest scale and smallish (by Angkor's standards, at least) structures. Judging by fragments scattered around, the temples were of brick. They were small enough to have been completely subsumed by vegetation in subsequent centuries, to the point that they now exist just as bulges in the earth. Some of them could only be spotted through surveillance from the air, which Aymonier obviously didn't have.
This all just goes to show how much there remains to learn about Angkor and the Khmer civilization. It's great that there's such an energetic team of people carrying on the work.