Mount Agung in Bali has been thrusting ash thousands of feet into the sky for almost two weeks. Lava is burbling at the volcano’s peak. Indonesian authorities have ordered evacuations around Agung, while tourists are stranded at the closed airport. The volcano’s flanks are bulging from magma trying to push its way out, and earthquake frequency has been increasing. Heat from the magma has melted snow and ice at Agung’s summit, causing volcanic mudflows called lahars. It’s looking like an eruption is imminent. But how do volcanologists know for sure what’s to come?