The most active volcano in the Philippines is well known for its perfect cone shape—which actually signals how dangerous the volcano can be. Its geology and location could soon combine in explosive, and deadly, fashion.

Mount Mayon, which is located on the big island of Luzon, has been spewing ash, rocks, and mud since January 13 as it prepares to erupt in what is expected to be a big way. On Monday, the Philippine Institute of Volcanology and Seismology raised the alert level to four out of five possible levels, meaning a sizable eruption could happen at any moment.

The alert went out after a loud explosion was heard, indicating high activity. Previous eruptions, like in 2006 and 2009, simply oozed