Srinagar, Aug. 6 -- Priests press pale, leafless stalks between heavy stones as altar fires rise into the smoke. The golden liquid they collect, filter through wool, and mix with milk or honey becomes the most celebrated substance in the entire Rigveda.

Three millennia later, historians still cannot agree on what that liquid actually contained, and the not-knowing has become a discipline of its own, drawing botanists, archaeologists, linguists, and neuroscientists into one of the longest-running investigations in religious history.

The puzzle of Soma is not only which plant produced it. It is why an entire civilization organized its cosmology around a substance whose physical identity disappeared from memory almost as fast as the hymns ...