JAPAN: The radioactive carbon isotope 14C decays at a predictable rate. By measuring the carbon in ancient organic materials, you can tell how long the 14C has been decaying, and therefore how old the object is—radiocarbon dating. But the atmospheric concentration of 14C has not always been constant, so knowing past 14C concentrations is essential to refining accuracy. Tree rings, corals, and marine sediments are used for this. Now researchers have another resource—the sediments at the bottom of Lake Suigetsu are so clearly layered, year by year, that they will help improve the accuracy of radiocarbon dating for objects between 10,000 and 52,000 years old. —Samir S. Patel