Researchers just released an update to the definitive list of most threatening volcanoes in the United States. It’s the first update for the volcano rankings, which were originally published by the United States Geological Survey (USGS) in 2005.

“There are a lot of volcanoes in the US, and they erupt more frequently than most people know,” says Angie Diefenbach, a geologist at the USGS and a co-author of the report. “We have a lot of volcanoes and there aren’t limitless resources. It helps to prioritize where we’re going to be working.” The United States has 161 active volcanoes, and it has experienced 120 volcanic eruptions since 1980.

The highest priorities for geologists like Diefenbach are the 18 volcanoes ranked as very high threats. Those haven’t budged from their position at the top of the heap since the original 2005 report. Leading the list are infamous volcanoes, including Kilauea, which has been continuously erupting in Hawaii since 1983, and Mount St. Helens, which erupted dramatically in 1980. The top 18 also include lesser-known peaks, including five in Alaska.

Beyond the top 18, the other 143 volcanoes on the list are rated from a high threat to very low threat, with many of the active volcanoes hovering in the middle. One important thing to keep in mind when scrolling through the rankings is that this doesn’t mean that any of these volcanoes are guaranteed to erupt anytime soon.

“This threat ranking is not an indication of which volcano will erupt next,” says Diefenbach. “The ranking is an indicator of the potential severity of impacts that can result from future eruptions at any given volcano.”

“There are a lot of volcanoes in the US, and they erupt more frequently than most people know.”

What makes a volcano threatening is a pretty simple formula, explains Diefenbach. Volcanologists rank volcanic threats by figuring out how the volcano might erupt — whether it will likely explode, like Mount St. Helens, or let loose a flow of lava, like Kilauea. Then, they figure out if people, property, or critical infrastructure are located in the potential danger zone. If so, they get ranked as a higher threat. That’s why Mount Rainier, only 59 miles away from Seattle, is considered a very high threat.

Out in the less-populated reaches of Alaska, a different calculation comes into play. While there aren’t many people directly threatened by Alaskan volcanoes, there are a lot of planes that cross through Alaskan airspace. Volcanic ash can severely damage jet engines, so air traffic controllers will often ground or redirect planes during a volcanic eruption, as they did in 2010 when an Icelandic volcano erupted. The risk to aviation, coupled with high levels of volcanic activity, is why five Alaskan volcanoes earned a place in the top 18.

Thanks to our high-flying travel patterns, “there really is no such thing as remote volcano when it comes to hazards,” says Diefenbach.