Beneath the stalactites of Laang Spean, in Battambang province, archaeologists have spent more than sixty years digging through a memory that stretches back several millennia. What the earth reveals there upends the idea of a fixed Khmer identity: Cambodia's first inhabitants did not all spring from the same soil. They arrived in successive waves, from the forests of Sundaland, the rice fields of southern China, and eventually the shores of India.