A cloud of dust up to 2,000 km long was tracked by scientists at the US National Aeronautical Space Administration (NASA) as it left Asia, drifted across the Pacific Ocean, and traversed North America from Alaska to Florida, raining dust and possibly pollutants over the continent. Dust clouds blowing east from Asia are a common occurrence in the springtime, but last month's million-tonne dust cloud was surprisingly large and long lasting. Pollutants too small to see have also been carried across the Pacific Ocean by the dust cloud.