Extensive excavations by ISIS militants under Mosul’s ancient Mosque of Jonah show they took care to preserve artifacts for loot, a local archaeologist said, in sharp contrast to their public desecration of antiquities.

The militants seized the mosque when they stormed through northern Iraq three years ago, bulldozing and dynamiting ancient sites and smashing statues and sculptures, declaring them all idolatrous.

Jonah’s mosque was blown up in July 2014, but experts surveying the damage after it was recaptured in January by a U.S.-backed Iraqi campaign found a network of tunnels dug by the militants, leading down to a 7th century BC Assyrian palace.

The careful way the tunnels were dug show the militants wanted to keep the treasures intact, said archaeologist Musab Mohammed Jassim, from the Nineveh Antiquities and Heritage Department,

“They used simple tools and chisels to dig the tunnels, in order not to damage the artifacts,” he said, standing near the tunnel network which leads from the mosque ruins above ground to the much older subterranean palace.

The digging “was carried out according to a plan and a knowledge of the palace,” he added.

The efforts to avoid damaging the antiquities contrast with the destruction of ancient sites across ISIS’s self-declared caliphate in Syria and Iraq, from the desert city of Palmyra to the Assyrian capital of Nimrud, south of Mosul.

The desecration was recorded on video and widely published by ISIS supporters, who portrayed it as part of their campaign to erase any cultural history which contravenes their extreme interpretation of Sunni Islam.

However the United States has said looting and smuggling of artifacts has been a significant source of income for the militants. In July 2015 the U.S. handed Iraq a hoard of antiquities it said it had seized from ISIS in Syria.

-REUTERS