In 1968 R. Gordon Wasson argued that the soma of the Rig Veda, the plant god pressed and drunk by the Vedic priests, was the fly agaric. The book was influential enough that the identification is now often repeated as settled. It is not.
The case for it: soma is described as growing on mountains, as having no leaves, root, blossom or seed, and as being pressed for a juice rather than crushed as a whole plant; it is golden or ruddy in colour; and one much discussed passage appears to refer to soma passing through a man and being urinated. Wasson read that as the Siberian practice, which would be striking if right.
The case against is substantial. The Vedic texts nowhere describe the white flecks, which are the mushroom's single most obvious feature and hard to imagine a poet omitting. The fly agaric does not grow in the regions where the Vedic hymns were composed, requiring an argument about earlier northern homelands that carries its own problems. The urine passage is contested in translation. And the effects the hymns describe, exhilaration, expansiveness, strength, do not match a mushroom that mainly produces nausea and sleep.
Rival candidates include ephedra, favoured by continuity with the modern Parsi haoma ceremony, and Peganum harmala, argued at length by Flattery and Schwartz. None of the three is established. The atlas records the candidates and the arguments, and leaves the question open, because it is open.