Volcanically, Antarctica is a fairly quiet place. But sometime around 325 B.C., the researchers said, a hidden and still active volcano erupted, puncturing several hundred yards of ice above it. Ash and shards from the volcano carried through the air and settled onto the surrounding landscape. That layer is now out of sight, hidden beneath the snows that fell over the subsequent 23 centuries.

Although out of sight, the layer showed up clearly in airborne radar surveys conducted over the region in 2004 and 2005 by American and British scientists. The reflected radio waves, over an elliptical area about 110 miles wide, were so strong that earlier radar surveys had mistakenly identified it as bedrock. Better radar techniques now can detect a second echo from the actual bedrock farther down.

The thickness of ice above the ash layer provided an estimate of the date of the eruption: 207 B.C., give or take 240 years. For a more precise date, Mr. Corr and Dr. Vaughan turned to previous observations from ice cores, which contained spikes in the concentration of acids, another byproduct of eruptions. Scientists knew that an eruption occurred around 325 B.C., plus or minus a few years, but did not know where the eruption occurred. “We’re fairly confident this is the same eruption,” Dr. Vaughan said.

Now, they know both time and place.

“It’s probably within Alexander the Great’s lifetime, but not more precise than that,” Dr. Vaughan said.

The under-ice eruption was probably similar to one in Iceland in 2004. Although explosive, spewing ash more than seven miles in the air, the Iceland eruption was much less powerful than Mount St. Helens, the volcano in Washington State that blew off its peak in 1980.