Archaeologists have discovered a 6.5-foot-high door threshold in the ancient Iraqi city of Nimrud. The threshold was found during the first major dig in the area since it was extensively destroyed by the Islamic State (ISIS) in 2016.

The door sill was found in the palace of the Assyrian king Adad-Nirari III, who reigned from 810 BCE–783 BCE.

Michael Danti, the archaeologist leading a team from the University of Pennsylvania, called the find “significant” after excavations began in mid-October. “Not only because it survived the Babylonian siege and destruction by ISIS intact but also because of its size,” he told the Art Newspaper, who first reported the news. “I’ve seen tablets that are smaller than one of the sign forms [cuneiform letters] on this slab.”

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British archaeologist Austen Henry Layard first excavated the Adad-Nirari palace in the 1840s. Since then, several slabs inscribed with the royal ancestors of the king have been found in the area. The slab that Danti and his team found was one of two excavated by Layard that had not been relocated to the British Museum, then likely re-buried after the collapse of the palace.

It was like “forensic archaeology,” Danti told the Art Newspaper. “We had to re-excavate the old excavations.”

Danti knew what the slab was from reading Layard’s books as a young student and recalling them during the excavation work. Another striking thing about the discovery of the door sill is that “ISIS might well have been aware of its existence,” Danti said. “And yet it was so well preserved.”

The vast majority of the excavated areas of the ancient city of Nimrud were destroyed by ISIS through multiple attacks on the area. While buildings, statues, and the site’s iconic ziggurat were destroyed, debris was used to bury some of the buildings nearby. As a result, Danti and his team now have to navigate layers ofdestruction and construction in order to excavate and reconstruct thousands of years of Iraqi history, including modern elements that had been added in the mid-20th century. He described the work as being “like a jigsaw puzzle.”

Earlier this year, Danti and his team at the Iraq Heritage Stabilization Program also uncovered several ornate Assyrian marble carvingsdating back around 2,700 years.