An airline crew flying into Yellowknife a few weeks ago reported seeing some unidentified high-altitude lights near the airport — apparently leaving both the crew on board and air traffic controllers on the ground perplexed.

Scientists found an enriched uranium particle over the Aleutian Islands and don’t know where it came from.

Ruthenium-106, an artificial isotope, was detected in early October and is now gone. European safety officials say it poses no health risk to residents and that it might have come from Russia.