Stalagmites form on the floors of some caves.
Like tree rings, the bands in a stalagmite represent periods of growth. But unlike tree rings, these bands don't form every year. Stalagmites grow only when the weather is warm and there is ample rain. To date a section of a stalagmite, scientists compare the relative amounts of certain uranium and thorium atoms. The older the section, the greater proportion of thorium it will contain, because the uranium atoms decay into thorium.