Imagine a speck so small that fifty of them would fit across a pinhead. Now imagine that, forty years after the worst nuclear accident in history, it is still dangerously radioactive. Such specks, between 8 and 50 micrometers wide, still lie scattered through the soil around Chornobyl in Ukraine. Visitors to the area still need protective clothing.

Why do these grains matter so much? Because they are frozen records of the disaster. Tobias Weissenborn, a physicist and doctoral candidate at Leibniz University Hannover, describes three classes. Some particles still look much like the original fuel, uranium dioxide.

Others are wrapped in, or fused with, zirconium, the tough metal that coated the fuel; they were formed when the molten fuel met it at extreme temperatures. The third kind was born when the reactor's graphite moderator caught fire and burned for ten days. In that blaze, the fuel oxidized into compounds such as U3O8, which is brittle and crumbles easily into fragments. Wind can then carry them, and breathing them in is a serious hazard.