Parts of the Western Wall are eroding 100 times faster than others, potentially undermining the stability of the ancient Jewish holy site, according to a new study, indicating it might be in danger of collapse hundreds of years in the future.

The Western Wall is a remnant of the Jewish Temple compound in Jerusalem’s Old City, dating back more than 2,000 years. The structure, a retaining wall of the Temple Mount, is the holiest site where Jews are allowed to pray. The Temple Mount above is off limits to non-Muslim worshipers for security and religious reasons.

While the newfound erosion could make for larger cracks and crevices for visitors to press prayer notes into, the study is bad news for those who hope to keep the Western Wall standing for another 2,000 years.

This sort of risk is measured in decades, and the scientists don’t predict real trouble for centuries, but they say sudden collapses are possible.

The stones that are eroding more quickly are made of fine-grained limestone that crumbles more readily after exposure to water, the study finds.

“Rainwater gets into the stones and causes dissolution. It’s similar to what happens to a sugar cube when it’s dunked in coffee,” said Dr. Simon Emmanuel, an earth scientist specializing in the interaction of water and rock, who conducted the study along with earth sciences doctoral student Yael Levenson at the Hebrew University of Jerusalem. “The stones that are made of finer crystals fall apart much more easily.”

The findings, published in July in the journal Geology, could be a reference for helping to preserve the wall, the researchers say – perhaps by treating the stones with a binding material.

Built along with the Second Temple by Herod the Great, a Roman client king of Judea, the remains of the Western Wall now reach seven levels of stone above the ground. The stones above those rows are more recent.

The huge Herodian-era stones receive the prayers millions of religious pilgrims every year, many of whom follow the tradition of placing notes within their crannies. In 2014, famous figures like Pope Francis and pop star Justin Bieber paid their respects. But anyone who visits the site can see that some of the stones are much more eroded than others.

To measure the differences, the Israeli researchers used a laser scan of the wall to create a 3-D map of its surface. In the map, they compared four heavily worn stones to their well-preserved neighbors. While the preserved stones were hardly eroded at all – as evidenced by the visibility of their original finely chiseled borders – the worn stones had shrunk by as much as dozens of centimeters.

The researchers couldn’t take samples from the wall, for obvious reasons, but they benefited from the volumes of research that has been done on the site. The eroded stones are thought to have come from one ancient quarry where the limestone is composed of tiny crystals, while the preserved stones are thought to have come from another quarry, where the limestone is made of larger crystals.

The researchers collected samples from near the ancient quarries, located in Jerusalem, and used a powerful atomic force microscope to see how the different rocks disintegrated when they came into contact with water. They found that the fine-grained rocks rapidly lost tiny particles from their surface, likely explaining the erosion of the stones at the Western Wall.

A microscope image of Jerusalem limestone made up of tiny crystals. (photo credit: Dr. Simon Emmanuel/Hebrew University)

A microscope image of Jerusalem limestone made up of tiny crystals. (photo credit: Dr. Simon Emmanuel/Hebrew University)

The experiments were designed to simulate the way rainwater interacts with limestone in nature.

The researchers say the average erosion rates they calculated would not endanger the wall for at least several hundred years, although it is possible that catastrophic erosion could happen at any time.

“It looks like Herod might have been the victim of shoddy contractors,” joked Emmanuel. “He was an ambitious builder, and there was intense demand for limestone at the time. Some corners may have been cut.”

The process of accelerated erosion seen in the study, which involves chemical and mechanical forces, has not been documented before. While physical forces are known to act on large rock formations, the researchers showed for the first time that water acts to shape rocks even on the micron scale.

The findings could help guide the development of more effective preservation techniques, they say. In particular, they say, it may be possible to develop materials that slow the rate of erosion by binding the crystals within rocks together. Such engineering techniques could be applied not just to the Western Wall, but also to other ancient religious and historical heritage sites in Israel and around the world.