The Nubian Aquifer, the font of fabled oases in Egypt and Libya, stretches languidly across 770,000 square miles of northern Africa, a pointillist collection of underground pools of water migrating, ever so slowly, through rock and sand toward the Mediterranean Sea.

The aquifer is one of the world’s oldest. But its workings — how it flows and how quickly surface water replenishes it — have been hard to understand, in part because the tools available to study it have provided, at best, a blurry image.

Now, to solve some of the puzzles, physicists at the Department of Energy’s Argonne National Laboratory in Illinois have turned to one of the rarest particles on earth: an elusive radioactive isotope usually ricocheting around in the atmosphere at hundreds of miles an hour.

Their first success was in distilling these elusive isotopes, krypton 81, from the water in the huge Nubian Aquifer, part of which lies two miles below the oases of western Egypt where temples honor Alexander the Great. Their second was in holding these isotopes still and measuring how much they had decayed since they last saw sunlight.