You might think you could just light stumps on fire and let them burn until they disappear. But since they're underground, there's no source of oxygen to sustain the flame. Even with kerosene soaked into the wood, the part of the stump under the surface won't burn. Gunpowder, on the other hand, burns even inside a sealed space because it contains its own source of oxygen in the form of potassium nitrate, or KNO3, better known as saltpeter. Get saltpeter into the stump, and it supplies oxygen to combust the wood.