The Bayon temple stands upon a thick sand mound of 15m in height from the ground. In1933, EFEO excavated at the center of the foundation mound down to the base and found the Buddha statue. The sectional drawing of the Bayon temple by EFEO shows a vertical shaft at the center with dimensions of about 2.5m x2.5m with a depth of about 14m. To study the safety of the foundation mound, JSA (Japanese Government Team for Safeguarding Angkor) made a boring at the center of the shaft in 2009. The boring shows that the shaft is filled back with very loose sand of N<5 down to the base of the foundation. Another boring was carried out  from the surface of the third terrace at the outside of the central Tower. The results show a laterite block layer of 6m in thickness beneath sand stones at the surface followed by sandy mound with very large SPT N-values larger than N>100. The relationship between SPT-N values and water contents of the sample shows the larger SPT-N values with the smaller water contents. A Sampled soil was submerged into waer and gradually weakened and completely collapsed within 10 min. The Bayon tower stands upon the foundation sand mound with a thickness of about 15m. The climate season from May to October of Cambodia provides rainwater to weakening the sandy mound. However, the amazing fact is the sandy mound foundation has been stable since construction for more than 700 years. The paper provides the amazing mechanism of the Angkor Sand.