See Amos Frumkin, Aryeh Shimron and Jeff Rosenbaum, “Radiometric Dating of the Siloam Tunnel, Jerusalem,” Nature 425, September 11, 2003, pp. 169–171. Scientists use the uranium-thorium radiometric dating technique to establish the age of calcite (calcium carbonate) in coral, shells and especially cave and spring travertine, and to a somewhat lesser extent for apatite (calcium phosphate) and related materials in bone and teeth. What allows the uranium-thorium “clock” to work is that two isotopes of uranium, 234U and 238U, spontaneously decay to an isotope of thorium, 230Th. When calcite or apatite precipitate, they always take on some impurities such as uranium—but very little thorium. Once precipitation occurs, the uranium-thorium clock starts ticking. Atoms of uranium start converting to atoms of thorium at a constant rate. As time passes, there are progressively fewer 234U and 238U atoms and correspondingly more 230Th atoms. The ratio of 230Th/(234U + 238U) is the measure of how much time has elapsed. When the calcite or apatite first crystallizes, the ratio is equal to zero; over time the ratio increases. The uranium-thorium clock is useful for dating materials between one thousand and a few hundred thousand years old.—Ed., with thanks to James Harrell, a geologist at the University of Toledo.