Every time in recorded history that Eyjafjallajökull volcano has erupted, the much larger Katla volcano has also erupted. Scientists are watching Katla carefully.
The last time Eyjafjallajökull erupted, it continued belching the Earth's unsettled insides for 14 months, from December 1821 to January 1823.
Scientists do not expect Eyjafjallajökull to keep northern Europe's airports closed for 14 months, but they suggest that Eyjafjallajökull's impact on world travel might not end with the end of this current eruption.
Moreover, Iceland's "Angry Sister" hasn't even awoken yet. The three times in recorded history when Eyjafjallajökull has erupted, its neighbor, the much larger Katla, has followed suit.
Data do not yet suggest that a Katla eruption is imminent. Yet, in some respects, it is the far greater concern, both in Iceland and beyond.
Katla has erupted 16 times since 930, in 1755 exploding so violently that its ash settled on parts of Scotland. In 1918, Katla tore chunks of ice the size of houses from the Myrdalsjökull glacier atop it, sending them careening down its slopes and into the Atlantic on floods of melted glacier water.