What makes the Yellowstone supervolcano especially dangerous is the fact that it’s not actually a volcano but rather a caldera. Unlike the classic cone-shaped volcanos we normally model our childhood science projects on, calderas are actually inverse volcanoes, looking more like crater-shaped, sunken volcanos.
Piton de la Fournaise : "Peak of the Furnace" is a shield volcano on the eastern side of Réunion island (a French department) in the Indian Ocean. It is currently one of the most active volcanoes in the world, along with Kīlauea in the Hawaiian Islands (Pacific Ocean), Stromboli, Etna and Mount Erebus in Antarctica. A previous eruption began in August 2006 and ended in January 2007. The volcano erupted again in February 2007, on 21 September 2008, on 9 December 2010, which lasted for two days.
Mount Shishaldin / ʃ ɪ ˈ ʃ æ l d ən / is a moderately active volcano on Unimak Island in the Aleutian Islands chain of Alaska. It is the highest mountain peak of the Aleutian Islands. The most symmetrical cone-shaped glacier-clad large mountain on earth, the volcano's topographic contour lines are nearly perfect circles above 6,500 feet (2,000 m).
Kīlauea is the youngest and southeastern most volcano on the Island of Hawai‘i. Topographically Kīlauea appears as only a bulge on the southeastern flank of Mauna Loa, and so for many years Kīlauea was thought to be a mere satellite of its giant neighbor, not a separate volcano.
Etna is one of the world's most active volcanoes, having been in a state of near-continuous eruption for half a million years. Documentation extends back to 1500 B.C. Etna's eruptions have caused very few deaths—hence its friendly-giant reputation—because the lava flows are slow, the eruptions are not particularly explosive, and preceding earthquakes usually give local residents plenty of warning.
The eruption of Krakatoa, or Krakatau, in August 1883 was one of the most deadly volcanic eruptions of modern history. It is estimated that more than 36,000 people died. Many died as a result of thermal injury from the blasts and many more were victims of the tsunamis that followed the collapse of the volcano into the caldera below sea level.
The volcano is one of the most dangerous on Earth thanks to its numerous explosive eruptions—and the city of Naples, which is slowly crawling up its flanks. The fact that it doesn't fall at the top of this list (heck, it's not even the most dangerous in Italy) betrays how hazardous the other volcanoes might be.