The Temple of Bel, or Baal, in the ancient city of Palmyra was bombed by ISIS on Sunday, according to The New York Times.

"Our sources confirmed hearing the sound of a huge explosion, but within the temple walls," activist Khaled al-Homsi said, The New York Times reported. "It's not a political battle, but this is a cultural battle, and everybody should participate in defending this heritage, this civilization."

The temple was a well-preserved piece of history with a still-standing altar, now believed to be destroyed by the Islamic State. Palmyra's buildings and ancient sites have been attacked by the terrorist group before. 

The Temple of Bel is 2,000 years old. ISIS has held Palmyra for about three months now and recently destroyed the Temple of Baalshamin in the same area. The ancient city of Palmyra is a UNESCO World Heritage site. The columns of the structure and walls are still standing, according to CNN, but the full extent of the temple's damage is unknown at this time. 

ISIS has destroyed artifacts and ancient structures in the past. This is because it finds "pre-Islamic religious objects or structures sacrilegious," wrote the chairman of Cornell University's Department of Classics, Sturt Manning, in an op-ed for CNN.

The Temple of Bel holds great historical significance in the region, according to The Guardian. "This temple is the most important one in Syria because of its size, its level of preservation, its architecture and inscriptions," said Maamoun Abdulkarim, Syria's director of antiquities.