IT COULD HAPPEN
Imagining the worst at four U.S. volcanoes, where the incredibly unlikely is still very, very scary

Mount Rainier, Washington
THE NIGHTMARE: Lahars—fast-moving flows of volcanic mud—rip down the 14,410-foot peak's western slope into lowland suburbs. The flows might be triggered by earthquakes, lava, steam, or—worst possible case—an undetected slip in surface rock, giving the 5,500 residents of Orting just 40 minutes to evacuate once monitors pick up a slide.
AT RISK: More than 150,000 people who live in potential lahar pathways.

Yellowstone Caldera, Wyoming
THE NIGHTMARE: Though this deep-sleeping supervolcano blows big just once or twice every million years, a massive eruption could conceivably pulverize thousands of square miles with falling rocks and bury what's left with ash, killing everything. Gases and particulates released into the atmosphere would also alter global climate for years.
AT RISK: Much of the western United States.

Long Valley Caldera, California
THE NIGHTMARE: An eruption near the mountain-town getaway of Mammoth Lakes, where historically peaceful magma chambers have the potential to blow ash six miles skyward. Once the debris falls back to earth, it could form into pyroclastic flows speeding toward developments at 100 miles per hour.
AT RISK: Many a nice ski condo and—if the blast is on a peak winter weekend—some 35,000 lives.

Mauna Loa, Hawaii
THE NIGHTMARE: Molten lava rolls down the tropical mountain's northeastern flank and incinerates the city of Hilo, the Big Island's main port and commercial hub.
AT RISK: Hilo's 41,000 residents would have days to evacuate before the slow-moving lava arrived, but the city itself could be burned to nothing and—if lava reached all the way to the water—the port could be permanently shut down.