To turn grasses and leaf bits into sturdy walls, the rats use the only natural glue available: their own urine. “Packrats don’t drink water, so their urine is very viscous,” Cole said. “It crystallises and becomes solid.” Like hardened amber, the solidified waste is an ideal matrix to keep plant fragments intact for thousands of years.

When researchers find a new nest in a cave or an outcropping, they pick it apart like prospectors—an extremely smelly process—and analyse the plants they find for carbon-isotope ratios that identify them as members of a particular species.

If a climate shift occurs, changes in these norms are recorded in the contents of middens at different altitudes. During cooler periods, for instance, most vegetation migrates downhill. As warming occurs, the plants move back uphill; some even disappear from the middens altogether.

Because no layers of sea or ice sediments can be found in most dry inland regions, plants from the middens have been scientists’ only way of piecing together climactic records for much of the Western US.

Middens contain markers of other types of environmental change as well. After dissolving urine from 10,000-year-old nests near Yucca Mountain, Nev., Mitch Plummer, a hydrogeologist at the Idaho National Laboratory, found that the samples had surprisingly high levels of chlorine 36 isotopes.

New York Times / ELIZABETH SVOBODA