I. The problem: According to studies on Mediterranean deep-sea cores by Ninkovich and Heezen,' tephra from two violent volcanic eruptions must have fallen on Crete in prehistoric times. The first of these eruptions occurred at least 25,000 years ago, and its tephra apparently fell on all parts of Crete. The other was in the Bronze Age ("Minoan") eruption of Santorini (Thera) about 3,500 years ago, whose tephra (referred to in this paper as the "Minoan tephra") covered at least the eastern end of Crete, possibly to an average depth of about io centimeters (pl. 5, fig. I). The latter evidence lent substantial support to the theory, enounced a quarter of a century earlier by Marinatos,2 that the sudden collapse of Minoan civilization was a direct consequence of that eruption. Olausson,3 also on the basis of studies of deep-sea cores, disputes Ninkovich and Heezen's identification of the upper tephra; he claims that there actually are three or four tephra layers of different ages in the cores, the youngest of which is not less than 5,000 years old. Some archeologists, finding no signs of a layer of volcanic ash in their excavations, remained unconvinced that tephra from the Thera eruption even reached Crete in significant amounts. Moreover, according to the currently accepted archeological dating, the general destruction on Crete occurred toward the end of the Late Minoan I B stage, in about 1450 B.c. (or, at the very earliest, 1470 B.C.4), whereas the ruins on Thera have so far yield d only one example of Marine Style pottery typical of Late Minoan I B, and a very early example at that.' The problem, therefore, is to find traces of volcanic tephra on Crete, to identify it conclusively as t e product of the "Minoan" eruption, and if possible to determine the exact time of its fall with respect to the stages of Minoan culture as defined by pottery sequences.