Volcanoes are amazing portals to the hot, living interior of the Earth, but they're also dangerous. Even small-ish ones can have a global impact. Here are some explosive facts about volcanoes.

1. The volcanic explosivity index measures the strength and size of eruptions.

Created in 1982 by Chris Newhall of the United States Geological Survey and Stephen Self of the University of Hawaii, the VEI quantifies the strength of volcanic eruptions by measuring the volume of pyroclastic material spewed by a volcano, including volcanic ash, tephra (fragments of volcanic rock and lava), pyroclastic flows (fast-moving currents of gas and tephra), and other debris. The height and duration of the eruption are also factored in. The scale ranges from 1 to 8, and each step indicates a tenfold increase of ejecta. Fortunately, there hasn’t been a VEI-8 eruption in the past 10,000 years.

2. Wah Wah Springs sounds cute, but it was actually a prehistoric supervolcano.

One of the biggest eruptions ever occurred about 30 million years ago in what is today eastern Nevada and western Utah, when a supervolcano threw 3500 cubic kilometers of magma over an area of about 12,000 square miles. The eruption left behind deposits of debris 13,000 feet deep. Consider that the 1883 eruption of Krakatau, in Indonesia, was heard thousands of miles away—and yet it was a minor burp compared to Wah Wah Springs, a VEI-8 eruption.

3. When it comes to volcanoes, lava is the least of your worries.

Garden of the Fugitives, PompeiiLancevortex, Wikimedia Commons // CC BY-SA 3.0

Lava generally moves too slowly to be the biggest threat from an eruption—but that’s not the case with pyroclastic flows. These super-hot, fast-moving currents of gas and tephra did in history’s most famous volcano victims: the residents of Herculaneum and Pompeii. In 79 CE, the flow from Mount Vesuvius that hit Herculaneum was as hot at 500°F—enough to boil brains and vaporize flesh—while the later, cooler wave that hit Pompeii roasted people’s flesh but left their bodies intact. They were preserved by the falling volcanic ash.

4. There are several ways a volcanic eruption can kill you.

As Io9 recounts, flying shrapnel, scalding-hot seawater, falling into a lava tube, poisonous gases, and volcanic smog, or vog, can do you in.

5. There are three types of volcanic eruptions.

Magmatic eruptions involve the decompression of gas within magma that propels it forward. Phreatic eruptions are driven by the heat from magma creating superheated steam. Phreatomagmatic eruptions are caused by the interaction of water and magma.

6. Volcanologists continuously keep tabs on volcanic activity all over the world.

One of the many initiatives tracking potentially dangerous activity is the Global Volcanism Program of the Smithsonian National Museum of Natural History. It also puts out a weekly report in conjunction with the USGS that features a map. The International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) especially monitors the so-called Decade Volcanoes—16 volcanoes that are potentially hazardous due to their history of large, destructive eruptions and proximity to populated areas. Among them are Rainier, Sakurajima, Vesuvius, and Santorini.

7. Other planets and moons in our solar system have volcanoes.

Plumes on Io captured by the Galileo spacecraft NASA/JPL/University of Arizona

That Jupiter’s moon Io is volcanically active has been known since 1979, when Voyager 1 imaging scientist Linda Morabito discovered the first evidence of active volcanism on a body other than Earth. But it’s far from alone. For instance, while Mars’s volcanoes appear to be either dormant or extinct, evidence from the Venus Express spacecraft suggests that many of Venus’s volcanoes are active.

8. Sharks like to hang out in a Pacific volcano.

In 2015, scientists recorded video of sharks happily swimming around in the acidic, hot, ash- and gas-filled waters near the Kavachi underwater volcano in the Solomon Islands, which is a mere 66 feet below the surface. This suggests extremophiles may be even more diverse than we thought.

9. The USGS's all-time best-selling map features volcanoes.

"This Dynamic Planet" [PDF] features more than 1500 volcanoes, 44,000 earthquakes, and 170 impact craters, as well as the major, minor, and micro tectonic plates whose movement creates these features. About 60 of Earth’s 550 historically active volcanoes blow every year.

10. An early 19th-century volcanic eruption in the Pacific changed the world.

Gillen D’Arcy Wood argues in his book Tambora: The Eruption That Changed the World that the 1815 eruption of the Mount Tambora on the Indonesian island of Sumbawa, which created a massive sulfate dust cloud that fundamentally altered the planet’s climate for three years, led to such diverse impacts as the first worldwide cholera pandemic, expanded opium markets in China, the U.S.’s first economic depression—and Mary Shelley’s Frankenstein.

11. A volcano stamp sold Congress on the Panama Canal. 

Before the Panama Canal opened in 1915, rival proposals for an Atlantic–Pacific link included a plan to carve a canal through Nicaragua, which had a lot more fresh water and much less deadly malaria than Panama. It also has significant volcanic activity, and in the early 20th century, one of its stamps featured an erupting volcano. In 1902, just before a U.S. congressional vote, a pro-Panama Canal French engineer sent this stamp to all 90 senators to hype the volcanic threat in Nicaragua. Panama got the vote by a slim margin.

12. You can surf a volcano.

Nicaragua's Cerro Negro, a new and very active volcano that first erupted in 1850—and blown 23 times since, most recently in 1999—has black pebble-covered slopes you can surf down on a metal-bottomed wood board, if you're adventurous and also have a death wish. Intrigued? Here’s our seven-point guide to surfing volcanoes.

13. The most volatile area of Earth is a circle of volcanoes called the "Ring of Fire."

Located at the rim of the Pacific Basin, the so-called Ring of Fire is a nearly continuous chain of oceanic trenches and hundreds of volcanoes spanning some 25,000 miles that’s home to 75 percent of the world’s volcanic activity, with some 452 volcanoes (active and dormant), 90 percent of the world's earthquakes, and 22 of the 25 biggest volcanic eruptions in the last 11,700 years.

14. Volcanic activity has several warning signs.

According to the USGS’s Volcano Hazards Program, volcanologists keep an eye out for ground movements caused by magma forcing its way upward through solid rock, earthquakes resulting from this heaving, and changes in heat output and volcanic gases. Other indicators include cracks in the ground, small steam explosions, melting snow, and the appearance of new hot springs.

15. A 2010 volcanic eruption in Iceland delayed flights for weeks.

The Eyjafjallajökull volcano began erupting on April 14, 2010 and didn’t stop for six weeks, spewing magma, ash, and gas. Planes were grounded across Europe. Though the eruption was a small one, it had an outsized impact because it spread unusually far and stayed for an unexpectedly long time in the atmosphere thanks to the irregular shape of the tiny porous ash grains, as LiveScience reports.

16. NASA is training people for life on Mars on one of Earth's active volcanoes.

For several years, NASA has been simulating life on Mars on the slopes of Hawaii's Mauna Loa volcano, one of the Decade Volcanoes. Each year, a small team of adventurers who meet the basic qualifications for the NASA astronaut program live in a solar-powered geodesic dome. If they want to go outside, they have to put on space suits. Still beats trying to escape poisonous gases and pyroclastic flows.