Islamic State of Iraq and Syria militants are inside the gates of the ancient city of Palmyra, a UNESCO world heritage site, according to reports. Here's a look back at other historical sites lost to the world because of Islamic State fighters' wanton destruction: Ancient city of Nimrud Nimrud was a major city in the Assyrian empire, most significant between 900 and 612 B.C. The city had many precious artifacts, frescos and sacred texts, some of which had yet to be uncovered. It was a very unique archaeological site in an area steeped in cultural significance. In March 2015 the Iraqi Ministry of Tourism and Antiquities confirmed that the Islamic State had bulldozed Nimrud. "[The Islamic State] continues to defy the will of the world and the feelings of humanity," the ministry said at the time in a statement. "They violated the ancient city of Nimrud."

Khorsabad Palace The site of a beautiful palace built by Assyrian King Sargon II between 717 and 706 B.C. had carved stone reliefs complete with ancient paint. Museums in Baghdad, Chicago, Paris and Britain house some of the palace's carved stone reliefs.

On March 9, 2015, the Iraqi Ministry of Tourism and Antiquities said it received reports Khorsabad had been destroyed. "We have warned before and we warn again that those gangs and their sick ... ideology will continue to destroy and steal artifacts as long as there is no strong deterrent," they said in a statement. Hatra Alexander the Great's successors established a circular fortified city with double walls that became the capital of an early Arab kingdom. The 2015 razing of the city "outraged" U.N. Secretary General Ban Ki-moon. "The destruction of Hatra marks a turning point in the appalling strategy of cultural cleansing underway in Iraq," he said in a statement. Artifacts in Mosul Museum Iraq's second largest city contained a museum with 173 priceless artifacts, including large statues from Hatra, a UNESCO world heritage site. In February 2015 the Islamic State released a video showing militants taking hammers to the Assyrian statues' ancient faces. Mosul Library UNESCO called the destruction of the library and the burning of thousands of its rare books and manuscripts "one of the most devastating acts of destruction of library collections in human history." The February 2015 loss "adds to the systematic destruction of heritage and the persecution of minorities that seeks to wipe out the cultural diversity that is the soul of the Iraqi people," said UNESCO's Irina Bokova in a statement. The Crac des Chevaliers and other Syrian sites The site of an 11th century Crusader castle, the Crac des Chevaliers, is one of several Syrian sites that are being irreparably damaged in Syria's ongoing bloody civil war. Due to the ongoing conflict, it is difficult to confirm the full extent of the damage, but the United Nations warns that a "rich tapestry of cultural heritage" at four sites -- Palmyra, the Crac des Chevaliers, the Saint Simeon Church in the ancient villages of northern Syria and Aleppo, including the Aleppo Citadel -- "are being ripped to shreds." "All layers of Syrian culture are now under attack -- including pre-Christian, Christian and Islamic," said a March 20, 2014 U.N. statement. Jonah's tomb The reported burial place of the biblical prophet Jonah, inside a Sunni mosque known as Nabi Yunus in Mosul, was reportedly destroyed by Islamic State forces. The Islamic State released a July 2014 video they claim showed the mosque's destruction.