One Icelandic volcano's ash cloud grounded 10 million travelers across Europe

When Eyjafjallajökull erupted in April 2010, meltwater from its ice cap chilled the lava so fast it shattered into fine glassy ash, which the eruption plume carried across Northern Europe. Fearing the ash could damage jet engines, roughly 20 countries closed their airspace for days, stranding about 10 million travelers in the largest air-travel shutdown since World War II. The volcano's tongue-twisting name briefly became one of the most mispronounced words in the English-speaking world.

— Wikipedia contributors, 2010 Eruptions of Eyjafjallajökull — Iceland, March–June 2010