The newfound site is about 3.1 miles (5 kilometers) from Jericho, even then a metropolis, so a quarry "makes sense in the Jordan Valley, which was then a center of activity, agriculture, and building," Zertal said.
It would have been the largest open-ground quarry outside of Jerusalem.
The researchers found recesses in the columns where people placed oil lamps to provide light, as well as holes through which leashes for work animals could have been tied.
But the chamber's run as a quarry likely lasted only about 400 to 500 years. What came next is a bigger mystery.
The engraved crosses, dated to no later than A.D. 600, suggest that the artificial cave could have become a monastery, Zertal said, though he cautioned his theories are still premature.
Jodi Magness, an archaeologist and expert in early Judaism, said the crosses alone don't point to the existence of a church, since random pilgrims may have entered and made the carvings—a common phenomenon.
Churches at that time would have also had an altar, apse, and other "liturgical furniture," she explained, though it could be that such evidence has not yet been found in the cave.
But "it's certainly not far-fetched that Christian presence in the cave is associated with monastic activity, because the area is a hotbed of monastic activity," said Magness, who holds a senior endowed chair in the department of religious studies at the University of North Carolina and was not involved with the cave's excavation.
Hideout for Persecuted Christians?
Team leader Zertal also suggested that the quarry may have been used as a hiding place.
For example, before Roman Emperor Constantine I legalized Christianity in 313, Christians were often persecuted in the empire.
But UNC's Magness said the uncertainty of the carvings' dates makes it difficult to say whether Christians took refuge there.
"I would want to see some hard evidence that you can date that Christian presence to before Constantine," Magness said. "How do we know crosses [were] put on walls before 313?"
Zertal added that the Roman army symbol also means it's possible that Roman Empire soldiers hid here. "It's a perfect place to hold an army—a place nobody can see."
Yet Magness said there is no evidence that the Roman army quartered its troops in underground caves.
The "$64,000 question" now is why the unusual quarry was 32 feet (10 meters) underground, Zertal said. Most quarries are aboveground, so workers don't have to lift heavy rocks quite as far.
But UNC's Magness said underground quarries are not that unusual in the region.
She described a similar underground quarry near Jerusalem that had been used to make stone vessels, such as plates, mugs, and bowls. In the first century B.C. to the first century A.D., such quarries were common throughout the region.
It's also not unusual, she added, that a quarry would be reused for other purposes later.
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