Mayon began its increasing activity in May 1999, with hazard status associated raised to Alert Level 1. On June 22, 1999, Mayon emitted an ash column that rose to approximately 10 km above the vent; the status was later raised to Alert Level 2. The emission was recorded by the seismic network of the Philippine Institute of Volcanology and Seismology as an explosion that lasted for 10 minutes. No volcanic earthquakes nor other visible signs of abnormal activity were observed before the explosion.