Beneath a parking lot in Jerusalem, archaeologists have unearthed a moat that may have once been used to divide the ancient city, according to a recent study in the Journal of the Institute of Archaeology of Tel Aviv University.

Located on a narrow ridge south of the Temple Mount, the structure is 30 meters (almost 100 feet) wide and 6 meters (20 feet) deep. Its length hasn’t been confirmed, but the project appears to have been a major undertaking.

“There is no doubt that this is a monumental feature that required the quarrying of [around] 13,000 cubic meters [more than 450,000 cubic feet] of Meleke rock formation and completely altered the landscape of Jerusalem for many generations,” write the researchers.

In the 1960s, British archaeologist Kathleen Kenyon stumbled upon the structure but assumed it was a natural valley. Because she uncovered a middle section of the ditch, she didn’t see its striking edges, which researchers say are clearly man-made.

Even now, the moat’s purpose is uncertain, as is its age. The site hasn’t revealed any organic remains, which would be necessary for carbon dating. Researchers think it may have been built in the Iron Age around the ninth century B.C.E.—but without knowing for sure, understanding how it was used is challenging.

“In those days, it served to divide Jerusalem into two: the acropolis with the temple and palace to its north and the older city to the south,” lead author Yuval Gadot, an archaeologist at Tel Aviv University, tells Live Science’s Jennifer Nalewicki. “If it was cut in earlier times—something we cannot prove—then its purpose was to defend the city from the north.”

These theories introduce intriguing new ideas about the ancient city—and challenge previous understandings of its layout.

“In all our reconstructions of what Jerusalem looked like back then, we just have a continuous urban landscape from the Temple Mount down to [the] bottom of the City of David, but this discovery completely changes that picture,” co-author Yiftah Shalev, an archaeologist with the Israel Antiquities Authority. “Now there is a clear divide between the upper city—the sacred, royal and administrative city—and the lower city.”

Gadot thinks the moat may have served several functions simultaneously. It could have been both a secondary defense mechanism and an internal barrier that cordoned off sacred sites and elite populations. “Its role was to secure the elites, isolate them from the rest of the city, but also to symbolically mark that this is was a sacred precinct: The Temple, the palace were all isolated from the rest of the city,” says Gadot.

The discovery poses more questions than answers. The study notes that the “interpretation of the ditch as a pre-Iron Age fortification moat” relies on “circumstantial evidence,” and the researchers “refrain from reaching any conclusion until further information is at hand.”

The moat was likely used through the Hellenistic period, when it was ultimately filled in.