Archaeologists in Mosul, Iraq may have stumbled upon a momentous find that could shed light on the world’s first empire: a previously undiscovered 600 B.C. palace buried deep under a shrine demolished by ISIS, according to a report in The Telegraph.

The archaeologists accessed the ruins under the demolished Nebi Yunus shrine by using tunnels dug by members of ISIS, local archaeologists told The Telegraph.

“I can only imagine how much Daesh discovered down there before we got here,” archaeologist Layla Salih told the newspaper, using an acronym for the Arabic term for ISIS. She is supervising a team documenting the unearthed artifacts one by one.

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“We believe they took many of the artifacts, such as pottery and smaller pieces, away to sell. But what they left will be studied and will add a lot to our knowledge of the period,” she said.

The Telegraph reports this is believed to be the first instance of ISIS using tunneling to access ancient artifacts. The group has been known to sell looted artifacts, including ancient statues, coins, jewelry and mosaics, on the international black market.

But in this case, many priceless relics apparently remain intact. The tunnels led archaeologists to treasures such as a marble cuneiform inscription of King Esarhaddon, who ruled from 681 to 669 BC, as well as intricate Assyrian stone sculptures of a demi-goddess, The Telegraph reported.

The palace is believed to be tied to a long succession of kings from the Assyrian empire: built for King Sennacherib, renovated and expanded by Esarhaddon (681-669 BC), and renovated again by Ashurbanipal (669-627 BC).

Islamic State fighters blew up the revered Mosul shrine on top of the hidden palace in July 2014 during an aggressive offensive through western and northern Iraq.

ISIS militants have engaged in a systematic effort to destroy shrines throughout the region because they believe the sites violate the fundamental tenets of Islam.

The Nebi Yunus shrine contains what Muslims and Christians both believe to be the tomb of the Prophet Jonah, as he was called in the Bible, or Yunus, as he was known in the Koran. In stories from the Bible and Quran, he is swallowed by a whale.

News of the archaeological breakthrough comes after Iraqi government forces reclaimed the western half of Mosul from ISIS last week in a dramatic military conflict that caused thousands to flee the area for safety.

Iraqi security forces, backed by a U.S.-led coalition, have been battling to retake Mosul for months.