Dumarçay together with Professor Thanikaimoni had taken soil samples in 1974 and again in 1977 from trial trenches that had been dug into the hill, as well as from the plain immediately to the south.
These findings were endorsed by UNESCO in A New Perspective on Some Old Questions Pertaining to Borobudur compiled within the 2005 UNESCO publication titled "The Restoration of Borobudur".