Earth is showered constantly by particles called muons that are created by cosmic rays, and clever scientists are finding ways to use them as probes of dense objects, including a massive pyramid in Mexico and volcanoes in Japan. American researchers also have proposed using the energetic particles to detect smuggled nuclear materials in vehicles and cargo containers. Muons are formed when cosmic rays from deep space interact with the atmosphere. The particles, which strike earth's surface at the rate of about 10,000 per square meter per minute, pass through large amounts of rock or metal with ease, yet their charge makes them easy to track.