In a dry climate, like that of Egypt, papyrus is stable, formed as it is of highly rot-resistant cellulose; but storage in humid conditions can result in molds attacking and destroying the material. Library papyrus rolls were stored in wooden boxes and chests made in the form of statues. Papyrus scrolls were organized according to subject or author, and identified with clay labels that specified their contents without having to unroll the scroll. In European conditions, papyrus seems to have lasted only a matter of decades; a 200-year-old papyrus was considered extraordinary. Imported papyrus once commonplace in Greece and Italy has since deteriorated beyond repair, but papyri are still being found in Egypt; extraordinary examples include the Elephantine papyri and the famous finds at Oxyrhynchus and Nag Hammadi. The Villa of the Papyri at Herculaneum, containing the library of Lucius Calpurnius Piso Caesoninus, Julius Caesar's father-in-law, was preserved by the eruption of Mount Vesuvius, but has only been partially excavated.