Update 11:00 AM EDT

Indonesia’s Mt. Agung may be on the cusp of its first eruption since 1963, an eruption that killed more than 1000 people. Since late August, seismic signals have increased in frequency, and last Friday, officials raised the alert status to the highest level. Local officials have evacuated around 75,000 people from the area and warned resid ents to keep an 8- 12 kilometer d istance from the volcano.

However, it’s still unclear whether—or when—the volcano may erupt, scientists stress.

Although it has been stated the status of Beware (level IV) since 22/9/2017, not a guarantee it will surely erupt. — Dr Janine Krippner (@janinekrippner) September 26, 2017

Volcanic eruptions are all about pressure—as buoyant magma rises through the volcano’s plumbing, the change in temperatures and pressures allows gasses to escape, putting pressure on the magma. As the magma rises more and more gas escapes, the pressure rises, until…BOOM! Scientists monitor earthquakes at the volcano, the gas it expels, and possible deformation of its surface for signs of an impending eruption.

Increasingly frequent earthquakes as the magma moves are one of the main clues, and Mt. Agung’s seismicity has increased significantly in the last week or so . On Monday alone , seismometers recorded more than 600 earthquakes . Latest seismic activity graph from MAGMA Indonesia: pic.twitter.com/279tKjwlLU — Dr Janine Krippner (@janinekrippner) September 26, 2017 C ertain types of gas can tell scientists that magma is rising through the volcano’s “plumbing.” Carbon dioxide, for example, escapes magma earlier in its rise—so an increase in CO2 could indicate that the magma isn’t close to the surface, said Jessica Ball, a research geologist at the California Volcano Observatory . On the other hand, an increase in sulfur dioxide generally means shallower magma—and a possibly more imminent eruption.

Adding another layer of complications, gases escape at different rates from different type s of m agma, said Janine Krippner , a volcanologist at the University of Pittsburgh . Gas escapes more easily from runnier magma and less easily from thicker, sticker magma, which can affect how the volcano erupts. But scientists can’t see through rock and soil, so they can’t be sure what type of magma is rising.

And even with a record of past eruptions, there’s no guarantee that each eruption will occur with the same warning signs, Krippner noted on Twitter.

They can be totally different. What happened in the past is an indication of what can happen in the future not guarantee of what will happen https://t.co/OZOANRXMFF — Dr Janine Krippner (@janinekrippner) September 26, 2017