After President Bashar al-Assad’s forces recaptured most of Eastern Ghouta, the forces discovered an underground network of tunnels formerly held by the rebel forces.

These secret passageways were crucial for the rebels stationed here, as the labyrinth held hospitals and other military essentials, the South China Morning Post reported. It was here that rebels transported weaponry and ammunition across eastern Ghouta.

A city within a city…

35 meters deep underground tunnels, leading to Jobar, Ain Tarma, even Zamalca, dug by prisoners of rebels. Tunnels also lead straight to commanders homes, who can park cars underground & go straight to safety of their homes. #Syria #Jobar — Hala Jaber (@HalaJaber) April 1, 2018

The makeshift hospitals were stocked with surgery tables, medical gowns, hospital beds and first aid supplies for those injured during the fight against al-Assad.

“The network of tunnels connected ammunition warehouses under buildings places difficult to identify for the Syrian air force,” a military official said to SCMP.

Photos from the siege of eastern Ghouta and seven years of war in Syria, via @TheAtlPhoto https://t.co/7s5RRSMNoU pic.twitter.com/sHm0UgZfgZ — The Atlantic (@TheAtlantic) March 27, 2018

One of the entrances, according to SCMP, was the basement of a military headquarters that was once a school.

This discovery came after the Syrian army sieged the area due to a Russia-orchestrated evacuation deal that bused thousands of Syrian rebels and their families out of the area. The civilians and fighters were moved to northern Jarabulus.