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10. Ontong-Java Plateau, South Pacific

This is the biggest volcano you've never heard of. When it erupted 125 million years ago, it covered a region of the south Pacific Ocean the size of Alaska with basalt, in some places as much as 30 kilometers thick. It was so big, the eruption itself is thought to have lasted 6 million years. Scientists call this type of volcano a large igneous province (LIP). They are highly mysterious, and appear to form when huge amounts of hot magma well up from thousands of miles deep in the mantle, near Earth's core. There's a lot of debate as to whether LIPs erupt in huge explosions, or just ooze out in massive sheets of lava. Either way, mass extinctions have a tendency to occur whenever one of these things go off, so it's probably a good thing we've never seen one in action.

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Image courtesy of USGS

8. Grimsvotn, Iceland

Nothing says "explosion" quite like the mixing of searing hot magma with ice from a glacier. It's a common circumstance at Grimsvotn, a volcano buried underneath the Vatnajokull glacier in eastern Iceland that last erupted in 2004. Each time Grimsvotn erupts, huge amounts of liquid build up under the glacier until the pressure becomes so great that the water literally lifts up the glacier and escapes in catastrophic floods, called "jokulhlaups." You don't want to be around for a jokulhlaup. The flood that came after the 1996 Grimsvotn eruption discharged 50,000 cubic meters of water per second, making it briefly the second-largest river in the world. But that kind of thing doesn't faze Icelanders – these are the same folks who once sprayed seawater on a lava flow to keep it from engulfing a nearby harbor town.

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Image courtesy of NASA

6. Krakatau, Indonesia

In 1883, humanity witnessed what scientists call a "caldera-forming eruption" in Indonesia. In plain English, we call that a mountain blowing itself apart. At 200 megatons of explosive power, the eruption was four times more powerful than the biggest nuclear bomb ever detonated. Since the volcano and island were one and the same, there wasn't much left after the explosion rocked the Sunda Strait and sent 100-foot-high tsunamis and scalding ash flows ashore up to 25 miles away. In the ruined void the volcano left behind, a new island has been growing back (through a series of much smaller eruptions) and is now around 1,000 feet high.

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Image courtesy of NOAA/Dimitris Sakellariou

4. Santorini, Greece

Look at the small group of five islands known as Santorini, and it's clear something bad once happened there. In fact, the islands all were one, until an eruption bigger than Krakatau blew the place apart about 3,600 years ago. Ash deposits 100 feet thick have been found 19 miles in all directions from the caldera. Shown here is a wall of the volcano where you can see layers of ash, lava flows, pyroclastic deposits and other volcanic products. The ancient eruption is thought to have spawned the tales of the "Lost City of Atlantis" and perhaps even hastened the collapse of the Minoan civilization on the nearby island of Crete.

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Image courtesy of NASA

2. Tambora, Indonesia

Between dozens of volcanoes, the biggest earthquakes in the world, and devastating tsunamis, Indonesia has a lot of geology to worry about. And Mount Tambora, a huge volcano on the island of Sumbawa, is no exception. The mountain produced a gargantuan eruption in 1815 that produced an ash cloud so big, it canceled the summer of 1816 in North America and Europe. The eruption also killed between 70,000 and 90,000 people, making it the deadliest in human history. And the No. 1 volcano in geologic history is ... (you saw this one coming)...

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