When volcano experts say Bali's Mount Agung is "extremely likely" to erupt within hours or days, it is because of charts like the one above.
Stations routinely collect data on tremors within the mountain, with a spiky red line charting the rise and fall of Agung's mood day after day.
But two weeks ago the red line spiked skyward, which experts said was a telltale sign an eruption is imminent.
Emeritus Professor Richard Arculus from Australian National University said there was a high chance of the volcano erupting by the end of the week.
"Seventy to 80 per cent within days, probably 90 per cent within weeks to months, but I'm reserving that 10 per cent in case it doesn't happen — so the odds are on, but whether it proceeds to an eruption or not is still uncertain," he said.
But he said similar types of seismic crises have been known to stall in the past.
"The key here is that the number of earthquakes is increasing and the level of which they are occurring in the crust has continued to shallow so that's quite concerning and means it is more likely to erupt than not," he said.
About 35,000 people have fled a 12-kilometre exclusion zone on Indonesia's tourist island where hundreds of tremors have been felt in the past two days.
The chart below shows the number of these tremors — the big, black blotch illustrates where we are now.
The last time Mount Agung erupted was in 1963 when more than 1,000 people were killed and the global temperature dropped by a fraction of a degree.
Professor Arculus said technological advancements would hopefully prevent such deaths.
"Our ability to predict eruptions has improved dramatically since this last event, so we can hope such a death toll will not occur again," he said.
He said scientists were looking at the number of tremors to determine when the eruption may occur.
Tiltmeters are also used to determine if the volcano is inflating and the nature of the gases also help experts to decide how soon the eruption is.
"Once the volcano starts erupting ash into the air, that's a very bad sign," Professor Arculus said.
He said if the Mount Agung volcano does erupt, the ash plumes can move around the world and impact on air travel for weeks.
Mount Agung lies about 75 kilometres from Kuta Beach where thousands of Australians holiday every year.
The Department of Foreign Affairs and Trade is urging travellers to monitor the situation, follow instructions from local authorities and contact airlines if the volcano does erupt.