The question I keep getting when I tell people that I jumped into Nyiragongo, an active volcano in the Democratic Republic of Congo is: “What did you do to the mountain!?”

It’s an odd question, but it makes sense when you realize that Nyiragongo is suddenly undergoing a new, more active phase. And it’s got the volcanologists at the Goma Observatory worried.

New activity in the crater over the last few weeks has spurred the scientists into action as a new vent has opened up on the ledge just above the lava lake. This was preceded by a series of earthquakes that knocked large rocks off the crater walls that I had only recently scaled.

The ledge that the vent opened on is the one that I didn’t make it down to, but the rest of the team did. However, I did stand directly above the area in which it opened, but, at the time, I took no notice of the nondescript ground far below me. I had zero inkling that less then 10 days after I left, magma would boil out of a newly formed crack in the earth.

The vent has formed on the side of the caldera closest to the city of Goma at the base of the mountain. In addition, new fumoroles (gas and steam vents) have opened up on the flank of the volcano close to where lava erupted in the 2002 eruption. In that eruption, 147 people lost their lives and over a third of Goma was destroyed.

The eruption was not explosive like Mount St. Helens, but instead, effusive like Mount Kilauea in Hawaii. During the eruption of Nyiragongo, lava flowed down the slopes from cracks in the side of the mountain at speeds that reached 100 km/h. The chemical composition of the rock makes the lava extremely liquid.

The concern right now is that this active phase could be a signal that a new eruption is imminent. Since the 2002 eruption, the city has expanded, but still lacks basic infrastructure that would allow quick evacuation. Goma simply is not a city that can be evacuated fast.

When I was there, only a few roads were fully paved and those were within the city core. The sprawling conglomeration of tin roofed shacks and cinder block buildings stretched haphazardly in every direction. Despite the efforts to rebuild the city after the long civil war, it is functional but struggling. Evacuation is simply not an option. This is why observation and early warning are critical to survival in the city.

The proximity of the city to the mountain is why Nyiragongo has been declared a Decade Volcano. These are 16 volcanoes identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) as being worthy of particular study in light of their history of large, destructive eruptions and proximity to populated areas. Essentially, it’s a list of the most dangerous volcanoes on the planet.

The greater danger lurks under the water of Lake Kivu. Goma sprawls out along the northern shore of the lake and deep in its depths is a massive amount of carbon dioxide that is slowly building thanks to the volcano.

Think of it like a pop bottle with the top on. Open it up and a bit of gas may fizz up. Now, shake that bottle and pop the top. The resulting eruption of gas and liquid will end up soaking the room and you.

Lake Kivu is similar, only on a far more vast scale. The weight of the water above the gas-saturated deep layers acts as a top, keeping the gas in solution. However, if the concentration of carbon dioxide builds to a critical level, it could all come out of the lake at once.

This isn’t likely to happen any time soon as the concentration is still too low. The danger is disturbance. If the volcano erupts and lava makes it to the lake, an eruption of the gas, known as a limnic eruption, could take place. That eruption would spill titanic amounts of carbon dioxide into the areas around the lake and potentially kill hundreds of thousands of people.

It is an eerie feeling to see these pictures of the Nyiragongo crater and realize that I was standing on a volcano that was mere moments from stirring to life. And a terrifying feeling that I may end up returning to a city that is slowly being engulfed in a new eruption of burning, liquid rock.