Mosul has long been known for its religious diversity. Iraq's second largest city has been home to Persians, Arabs, Turks, and Christians of all denominations since it was first believed to have been settled in 6000 B.C. The ruins of Ninevah, one of the greatest cities in antiquity and former seat of the Assyrian Empire, lie within its modern city limits.

But now the Islamic State (IS) has arrived.

The Sunni extremists of the IS, previously known as the Islamic State of Iraq and Syria (ISIS), have been working to erase evidence of that diverse history since they seized the ancient city on June 10. (Related: "Iraq: 1,200 Years of Turbulent History in Five Maps.")

By some estimates 60,000 Christians lived in Mosul a decade ago, a number that may have been halved over the past decade of turmoil but could now be close to zero following an order by the IS to convert, leave, or die. This month reportedly marks the first time in 1,600 years in Mosul that no Sunday Mass has been held. (Related: "Iraq Crisis: 'Ancient Hatreds Turning Into Modern Realities.'")

The IS is also trying to eradicate visual evidence of belief systems that don't follow its strict interpretation of Islam. The Sunni extremist fighters have removed or destroyed more than a dozen tombs, statues, mosques, and shrines—including shrines that hold meaning for Jews, Christians, and Muslims alike—such as the site believed to be the tomb of the biblical prophet Jonah, which was wired with explosives and detonated last week. The shrine of Prophet Seth, considered to be the third son of Adam and Eve, has also been demolished.

Archaeologists, historians, and many in the local populace are distraught. Iraqi-British archaeologist Lamia Al-Gailani Werr is an honorary senior research associate at the Institute of Archaeology, University College London, and a senior researcher with the Department of the Languages and Culture of Near and Middle East at the University of London. Born in Baghdad and educated in Britain, Al-Gailani Werr has worked extensively in Iraq, previously serving as a consultant to Iraq's Ministry of Culture for Baghdad's Iraq Museum.

She spoke with National Geographic about the physical and spiritual heritage being lost in Mosul today.

Were you at the Baghdad museum when it was looted in 2003, and are there similarities between then and what is happening now in Mosul?

I went to Baghdad in June of 2003, after the looting. There is a difference between what happened then in Baghdad and now in Mosul—no standing building was destroyed in 2003. Back then it was the looting of antiquities from the Iraq Museum and the illegal looting of ancient sites. In Mosul, it is standing and mostly religious buildings that are the targets, and many of are of great archaeological heritage value.

Mosul is one of the oldest cities in Iraq. Ninevah is now part of the city; it used to be just outside Mosul. During the 9th century onward, Mosul was the seat for all the Christian religious movements and studies. Just outside the city is one of perhaps the oldest monasteries in the world—Mar Mattai, or St. Matthews.

View Images Mar Mattai, one of the oldest monasteries in the world, is in danger from the Islamic State's systematic attack on religious sites. Photograph by Michael Runkel, imageBROKER via Corbis

Is that monastery safe so far?

I have not heard anything about Mar Mattai, so it could be still safe. Some say it dates back earlier than the fourth century, to the second century.

The Assyrian Empire dates to the first millennium B.C., but there are a lot of sites within the area that go back 10,000 years. Mosul is absolutely rich with archaeological sites. Rich with people too: The people there count themselves as being in the center of the world. The people of Mosul are very proud of their city. For Christianity, the Eastern Church in Mosul was really the church that spread Christianity to the east. Islam was also there from the beginning, when it came through Iraq in the seventh century.

Did Mosul change significantly after the U.S. intervention in Iraq?

Mosul was always diverse. There are several sects living there, different offshoots of Islam or Christianity. One is called Shabak, which is an offshoot of Shiite Islam; they've been living there quite freely, quite peacefully together. But in 2003 the fundamentalists did start having a foot in Mosul.

I remember when I was in Baghdad in 2003 and 2004, I heard there were streets in Mosul that people called Kandahar [the religious and political base of the Taliban in Afghanistan] because there were all these people who were fundamentalists and were dressing in that Islamic style.

View Images Iraqi elderly men leave Mosul's Al-Noori Al-Kabeer mosque on July 9. The leader of the IS purportedly delivered a sermon there on July 4. Photograph by EPA

Have you visited the sites that have been destroyed?

I went to visit the archaeological sites in 2001. We saw Nabi Yunus [the tomb of Jonah], which has a mosque that has been renewed again and again. The minaret of Nabi Yunus was only from 1924 because the old one fell down. Nabi Yunus has been renewed quite often—during Saddam's time they did a lot of renovations. Mosul has a number of these shrines that go back to the 9th, 10th century, especially 12th and 13th century.

The shrine of Jonah, isn't that something of value not just to Jews and Christians but also to Muslims?

Yes, it has—or it had—a mosque over it. It's difficult to say when it was built, but Nabi Yunus stands on top of a mound that was probably an Assyrian temple. After the Assyrians it became a Zoroastrian temple. Then it became a church, and afterwards it became a mosque. In the 1990s, the State Board of Antiquity and Heritage did excavate at the bottom of this mound and they found the gates from an Assyrian palace.

Why is the IS destroying places that are also important to Islam?

They are shrines. The IS, or the fundamentalist Salafist people, don't think that it is right to go and worship a dead person. They are absolutely against that. So what they've been doing literally is destroying any shrine. Not mosques, but shrines. They did destroy mosques or smaller mosques that belong to the Shiites, but they consider the Shiites as not religious, as not Islamic.

The Shiite mosques are called husseiniya. The IS has been destroying them systematically, not only in Mosul but also other places. But then the minute they got to Mosul, they demolished a shrine which is from the 12th century. It was that of Ali ibn al-Athir, a historian and writer from that period who was accused even then of being an apostate.

Aren't they also, like the Taliban in Afghanistan in 2001, attacking relics that depict a human face or form?

We don't have that in these sort of shrines in Iraq, these human forms. That's mostly in Christian places. But they could destroy these in Christian places if they get to them.

The extremists also tried—and so far have failed—to destroy the crooked minaret of Mosul, which is said to be 840 years old.

Yes, it is still standing. Next to it there's another shrine, and they presumably were intending to destroy it. I heard that they put all these explosives around it and asked the people who live around it to evacuate their houses.

But the local people have shown complete opposition to them, and there's another militia that came in and surrounded the place so the IS people left. So it's been spared for the time being. We don't know what will happen next. This is the most frightening thing, that minaret.

View Images People walk on the rubble of Nabi Yunus, or tomb of the biblical prophet Jonah, which was destroyed by the IS on July 24. Photograph by AP

Why is that so frightening?

Because it really is more iconic to Mosul than even the tomb of Jonah. It's like the leaning tower of Pisa. All the Iraqis and the people of Mosul are so proud of it. It's a beautiful minaret. The Hadba Minaret is built of brick, which is all intricately decorated and is from about the 12th-13th century.

Why is the minaret crooked?

It is something to do with the geological ground—a structural fault. Most minarets in Iraq, and especially in Mosul, tend with time to lean slightly. That is one reason why minarets are always being replaced. However, the Hadba is still standing after so many centuries and has become the icon of Mosul.

What is the cultural value of these minarets and shrines?

They are very important. If we're talking about Islamic shrines, quite a number of them have very distinctive architectural domes. Because most of the domes in Iraq are built with brick, not many of them have survived. In Mosul, however, there are quite a number of them and they're being destroyed. From an architectural point of view, it's a great pity.

Has there been any other time in Mosul's history where its diversity has been so threatened?

Never like now where there is an evacuation of all of them [the Christians]. That was the lovely thing about Iraq—we lived all of us together, and it is politics that has interfered. This time it is fundamentalist Islam. I'm very angry about this. The Jews were in Iraq from Babylonian captivity. And then politics let them leave from the 1950s onwards, and now the Christians are going. I remember as a child my father had three childhood friends. One was a Jew, one was a Christian, one was a Muslim. That gives you a symbol of what it was like. (Related: "What Does It Mean to Be Iraqi Anymore?")