7 Volcanoes We Should Be Watching  Two years ago, Iceland's
Eyjafjallajökull brought most European air travel to a halt.
This year, volcanoes such as Mexico's Popocatépetl and
Indonesia's Lokon-Empung are rumbling. Volcanoes are impossible
to predict with certainty, but these are seven active ones that
we know we must watch.  By Laura Kiniry   Iceland's Katla  In
April 2010, Iceland's Eyjafjallajökull ice cap erupted in a
massive ash cloud, disrupting air travel across Europe and
costing airlines hundreds of millions of dollars. But
Eyjafjallajökull is one of the country's smaller volcanoes.
Experts say its neighbor, 4961-foot Katla, is the one folks
should worry about.  Katla is an active subglacial volcano with
a large ice-covered caldera (akin to a cauldron). Although it
hasn't had a significant eruption since 1918, Katla's eruptions
over the past 1000 years have typically followed those of
Eyjafjallajökull. Since Eyjafjallajökull's 2010 eruption,
experts have noticed a rise in Katla's harmonic tremor, a s