Tunnels run for hundreds of metres, connecting caves strewn with mattresses that formed what the Syrian army and its Russian allies say was a vast rebel underground network.
The road leading to the entrance of the tunnels in Lataminah in northwestern Syria is lined with the charred shells of cars and armoured vehicles.
According to the Russian army, which organised a press tour of the site for dozens of journalists, the network of caves dug into a rocky outcrop could shelter up to 5,000 people.
“We think this network was dug about four years ago with sophisticated machinery, of a kind which is not available in Syria,” a Syrian army colonel said as he led reporters into the tunnels, escorted by Russian demining experts.
The red-brick entrance to this underground base still bears the scars of the battle that saw Russian-backed regime forces retake the area in the province of Hama earlier this year.
“Those who fought here retreated to the north. First to Khan Sheikhun and then further into Idlib province when our forces took the city,” the colonel said.
In some places, the tunnels are barely big enough to stand in but connect large rooms carved out of the rock, including a prayer room, a drone workshop, a bathroom and even a prison.
Military officials told AFP reporters that the total size of the underground network, in which crates of ammunition were found, has not yet been fully assessed.
It was used primarily by fighters from jihadist groups, among them the alliance known as Hayat Tahrir al-Sham that now dominates the entire Idlib enclave.
The caves provided shelter to those fighters from the intensive airstrikes Russian and Syrian aircraft usually conduct as a prelude to any ground advance.
In some of the caves, empty food cans and crumpled plastic water bottles, jerricans and decaying clothes give a glimpse of daily life in the dark hideout.
Some rooms were done up with tile panels and a coat of paint while others have fully cemented walls, over which Syrian soldiers have since scribbled slogans praising President Bashar al-Assad.
One room was even equipped with an old TV, wired up with cables that run around one kilometre from the nearby town of Lataminah.
The room which officers believe was used as a prison was dug out no less than 400 metres deep into the maze of tunnels and caves.
Blood stains are still visible on the ground, as are tiny separate cells with rusting doors.
The Russian army said it has uncovered around 10 such underground networks across northwestern Syria and others in the desert region of Palmyra.
Officers said the Lataminah cave was a local hub for the manufacture of drones that jihadist fighters used against regime and Russian forces.
The massive Russian military base of Hmeimim, which lies in the neighbouring province, has been repeatedly targeted by rebel drone attacks.