Researchers said the volcano under Antarctica's western and most rapidly changing ice sheet occurred in 325 B.C. and the volcano remains active.
Using airborne ice-sounding radar, survey scientists discovered a large layer of ash produced by the volcano.
"The discovery of a subglacial volcanic eruption from beneath the Antarctic ice sheet is unique in itself," said Hugh Corr, lead author of the study. "But our techniques also allow us to put a date on the eruption, determine how powerful it was and map out the area where ash fell.
"We believe this was the biggest eruption in Antarctica during the last 10,000 years," he added. "It blew a substantial hole in the ice sheet, and generated a plume of ash and gas that rose around 12 kilometers (7.5 miles) into the air."
Study co-author David Vaughan said the discovery is another vital piece of evidence that will help determine the future of the West Antarctic Ice Sheet and refine predictions of future sea-level rise.
The discovery is reported in the February online edition of the journal Nature Geosciences.