The volcano that buried Pompeii in A.D. 79 still rumbles deep down. Last erupting in 1944, Mount Vesuvius poses an ever-present threat to the Italian populations around Naples. Whether the volcano will erupt in Pompeiian proportions again is a question that preoccupies scientists monitoring it, as they hope to predict when Vesuvius will blow and provide adequate warning time.

One way to gauge the magnitude of an impending volcanic eruption is to determine the size of its "throat"—the internal tube through which magma travels upward to the surface. The larger the tube, the bigger the eruption is likely to be.

Hiroyuki Tanaka of the University of Tokyo reasoned that the throat could be "x-rayed" with energetic muons produced in cosmic-ray showers. The number of muons passing through the volcano would depend on the density of intervening rock, so measuring the number of muons passing through various parts of the volcano could yield a crude, 3-D view of the interior.

This clip from "Deadliest Volcanoes," by PBS's NOVA, provides a look at muon imaging. The entire NOVA program explores the threats from volcanoes around the world, including the supervolcano below Yellowstone National Park and the eruption threat in the U.S. Pacific Northwest. It airs on Wednesday, January 4, at 9 P.M. on PBS.

Watch Sneak Peak: Deadliest Volcanoes on PBS. See more from NOVA.