ALEPPO — In this city, where battles to control each square block are often fought hand-to-hand, rebel factions are reverting to one of the oldest means of strategic warfare: underground tunnels. Above ground, rebels are exposed to the superior military power of President Bashar al-Assad"s forces. But in the tunnels, they say they are safe and can travel through them freely, without fear of regime bombs and snipers.

During the course of Syria's conflict, tunnels have been used strategically in a range of scenarios. They helped the rebels of Homs survive the regime’s two-year siege of the city. They used the narrow, dark crawl spaces to transport food and medical aid into the old city. In time, the tunnels were discovered and sealed off by the regime.

Now, rebels in Aleppo have adopted them as a military tool — with a bang, not a whimper. On March 18, a major explosion rocked Aleppo's old city — the result of a bomb planted in a tunnel dug under the Court of Justice, a temporary Assad military base that was one of the president's best-fortified buildings in the area. Residents described an earthquake-like blast that shook a wide swath of Aleppo, with plumes of dust filling the sky. In its immediate wake, clashes broke out between the rebels and forces loyal to Assad.

Minutes before the explosion, Sami, a fighter with the Free Syrian Army, ran through a maze of alleys in old Aleppo. The streets were empty, and ahead were larger buildings controlled by the Syrian regime.

When the explosion hit at the designated hour, Sami conveyed it to his friends over a walkie-talkie. “Target hit. The building is destroyed,” he said. When the dust settled, the building became visible. Half of it had been leveled. In the chaotic aftermath, rebel groups led by Jabhat al-Nusra, which maintains control over chunks of Aleppo, set up military checkpoints near its husk.

The building itself had been a major obstacle facing the rebels’ plan to advance against regime forces in the old city, housing the well-trained snipers the Assad forces had stationed at its windows. Destroying the building meant rebels could now control whatever was left. The momentum led them to take over military checkpoints in that area and further tighten the noose on the Syrian forces patroling the Aleppo Citadel.

Portrait of a tunnel digger

Sami, known as Abu Ahmad to his friends, had been one of the men who dug the tunnel, starting on a nearby street. “It took a long time and a lot of effort,” he says. “We took turns digging, another 11 men and me. We worked all day and all night to reach the Court of Justice.” The longest tunnels can reach up to 850 meters (930 yards) long, taking months to finish.

Sami says discretion is the main component of a successful underground operation. Diggers are carefully selected by senior rebels. “A lot of my brigade mates didn’t even know about the tunnel,” he says. “It was extremely secretive to ensure its success. I also know that my friends were digging other tunnels, but I didn’t know where they led to.” The bombs are planted and later detonated remotely.

Operations like Sami's have become commonplace, part of a rebel campaign launched March 14 and dubbed “the Aleppo Earthquake.” The title is in reference to the underground tunnel explosions, which reverberate like a series of smaller quakes.

Since then, the Hanono army base, the Carlton Hotel and the Chamber of Industry have all been targets of underground bombings. All had served as headquarters for Assad forces, and all had been targeted in a similar manner as the Court of Justice. Tunnels were dug and bombs were remotely detonated. “Even if they have air power, we are good at digging,” Sami says of the regime, which has waged an intense barrel bombing offensive on opposition-held areas of the city since last December. “And we will come to them ... from where they don’t anticipate.”

Digging these tunnels, Sami says, doesn’t require much experience beyond basic engineering and ancient warfare. A compass and a yardstick often get the builders to their target, while depth is measured against water and sewage pipes.

On June 2, the FSA's Fajr al-Hurriah brigade tunneled to and destroyed three buildings controlled by Assad forces on the front lines of the al-Midan neighborhood. “We only had diggers and a few explosives,” says Amer Hassan, the brigade's media director. “It’s not a complicated matter. No one helped us dig these tunnels.”

On a recent day near another of Aleppo's front lines, five FSA fighters were hunched over, digging a tunnel. Their tools were primitive: a compass, a meter, a spade, a pick axe and a cart to move the dirt. No powered machines were used, because, Sami says, “they make loud sounds and vibrations. We would be easily discovered if we used them."

Regime forces in the city are now more alert to the possibility of tunnels being dug below them. To prevent their success, Assad forces themselves have started digging, setting up deep trenches both along the Nile Street to Maysalon Bridge front line and around their army bases. They are working to prevent rebels from attacking from underneath their feet.