Lava domes are mounds of viscous lava and their collapse can cause deadly pyroclastic flows. This paper looks at the example of Mt. Unzen in Japan. Using novel experimental techniques, we discovered that crystals and bubbles in the lava make it behave differently to what was previously thought and that it becomes weaker and more susceptible to collapse as it cools. This calls for a review of current models, allowing for better failure prediction of lava domes in the future.