Thomas Cunningham, age 11 of Allentown, Pennsylvania, for his question:

How do scientists know about the earth's mantle?

The most solid evidence comes from earthquakes. Around the world, seismograph stations track their shivering vibrations. Some of them travel around the globe through the earth's crust. Others travel straight down through the globe. These are the ones that reveal information about the mantle below the crust. Vibrations, of course, change as they pass through materials of different densities.

Earthquake vibrations passing down through the earth change when they reach the lower limit of the lightweight crust. Their patterns reveal that the layer below is made of much denser materials. This layer maintains more or less the same density to a depth of 1,800 miles. It is, of course, the earth's mantle. At its lower level, earthquake vibrations change again and indicate that the central core is made of still denser materials.