When the Syrian army regained control of al-Qaboun district in Damascus on May 13, a network of tunnels stretching for kilometers under the city was discovered. Rebels used the tunnels not only in their fight against government forces, but also to facilitate a multi-million-dollar business of supplying armed groups in besieged areas.

Ruins are a common sight in many cities in today’s Syria. However, in al-Qaboun, the fierce fighting that took place above the ground is just a fraction of what was going on beneath the streets.

Ruined buildings in al-Qaboun. /CGTN Screenshot

The Syrian army said it discovered 26 tunnels underneath the al-Qaboun district after it recaptured the area from rebels. 

"The battle to retake Aal-Qaboun district took several weeks despite the fact that they dispatched their elite force – the Syrian army's fourth armored division and the republican guards. One of the reasons this battle was difficult are tunnels. According to officers in the Syrian army, there are several tunnels, in fact a maze of tunnels. And these tunnels allowed the insurgents to move undetected and ambush the army,” said CGTN’s Alaa Ebrahim.

Tunnels under al-Qaboun district in Damascus. /CGTN Screenshot

An ordinary-looking building or a pile of rubble could have many secrets hiding underneath it. The tunnels underground almost made a city beneath a city.

Since mid-2013, the Syrian government has worked to fully encircle rebel factions in Eastern Ghouta, one of the largest and most important rebel-held areas. But little did the government know, they were opening the way for a new type of trade with areas under the government's siege.

Soldiers of the Syrian army check the tunnels. /CGTN Screenshot

Tunnels were initially used to move weapons, but soon became the source of much-needed items in rebel lands near the capital. A liter of diesel fuel would sell for as much as 10 US dollars in besieged areas, ten times more than its worth at the other end of the tunnel. The tunnels under al-Qaboun weren't just used to move fighters and weapons, they were also a gateway for vast wealths.