In ancient times, two Roman towns, Pompeii and Herculaneum, lay close to a great volcano in what is now Italy. About 2,000 years ago, the volcano erupted, releasing volcanic ash that fell on the towns and buried them. Pompeii and Herculaneum vanished completely beneath the ash. When they were rediscovered and gradually excavated beginning in the 1700s, the towns were surprisingly well preserved, and they became important sites for archaeologists and tourists. Unfortunately, once uncovered, the towns began to decay. Several factors now threaten to destroy these remarkable sites. One problem is wild birds damaging works of art contained within the towns' structures. Ancient Romans created paintings on walls, and the ash covering Pompeii and Herculaneum preserved the paintings in excellent condition until the 1700s. However, now that the towns have been excavated, birds such as pigeons gather on the towns' walls and leave droppings. The droppings are acidic and cause paint to deteriorate, damaging the irreplaceable wall paintings. A second problem is damage caused by human tourists. Most tourists are respectful about preserving the sites when they visit, but less responsible tourists deliberately steal chunks of the ancient walls to take home as souvenirs. Others write graffiti on the towns' buildings. Both sites have security guards, but there are not enough of them to monitor all the tourists. Hiring enough guards to protect every wall and building would be impossibly expensive. Third, the buildings of Pompeii and Herculaneum have experienced structural failures. Although many of the buildings remained standing under the ash for nearly 2,000 years, they began to collapse when the ash was removed. In the early 1900s, engineers tried to prevent building collapses at these ancient sites by installing cement supports. The supports, however, have not worked well, and the structures continue to fall down