“We had scarcely sat down to rest when darkness fell,” wrote Pliny the Younger, “not the dark of a moonless or cloudy night, but as if the lamp had been put out in the closed room.” He was writing about the eruption of Mount Vesuvius, which he survived. The Italian city of Pompei, however, was submerged under solidified lava for almost 1,700 years, not to be discovered until 1748. A new exhibition juxtaposes the Italian ruins with those of Akrotiri, the ancient town on the Greek island of Santorini, which suffered a similar fate in the 5th millennium B.C. Until it was found in 1967, it, too, was frozen in time. —J.V.