It seems as though the thorium, a component of super nova debris, explodes as it enters Earth’s oxygen and water rich atmosphere. Perhaps the large hole that was made in the Russian lake was made by a piece of debris as small as a golf ball. A pyrophoric substance (from Greek πυροφορος, purophoros, “fire-bearing”) is a substance that will ignite spontaneously in air (cf. hypergolic). Examples are iron sulfide and many reactive metals including uranium, when powdered or thinly sliced. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or (with a few exceptions) nitrogen. Most pyrophoric fires should be extinguished with a Class D fire extinguisher for burning metals.