Mount Merapi, Indonesia’s most active volcano, erupted, emitting a river of lava onto the mountain below and gas clouds into the sky.

The eruption set off the volcano’s longest lava flow since the danger level for Merapi was raised in November, Hanik Humaida, the head of Yogyakarta’s Volcanology and Geological Hazard Mitigation Center said. Sounds of the eruption could reportedly be heard almost 18 miles away.

The 9,737-foot volcano sits on the densely populated island of Java and near the ancient city of Yogyakarta. It has repeatedly erupted recently, keeping local officials and residents living nearby on a state of alert.