by ELLIOT CARTER

On March 4 at 5:31 p.m., computer screens at the European Mediterranean Seismological Center lit up. A a 2.3-magnitude tremor had just rattled Aleppo in eastern Syria.

But it wasn’t an earthquake.

Rebel tunnelers had planted a huge stash of explosives under the Syrian Air Force Intelligence Directorate headquarters. The underground attack represented a powerful blow against the Syrian regime.

The explosion was a current adaptation of a medieval siege tactic. Pre-modern soldiers would dig a tunnel deep under an enemy castle’s walls, collapse the tunnel and bring down the castle along with it. Syria’s rebels do the same, but adding explosives to better collapse the tunnel.

Syrians captured the Aleppo explosion on video. The Syrian Observatory for Human Rights reported that 20 government soldiers and 14 rebels died in the blast and during an ensuing firefight.

Syrian rebels have long fought Bashar Al Assad’s regime on the ground. And now clever resistance fighters have opened another front … beneath the earth.

Tunneling is older than medieval — it’s actually an ancient battle tactic. One Syria’s rebels have eagerly adopted. Al Assad’s snipers and heavy weapons dominate the fighting above ground. It’s hard for rebels to make a move without the army noticing.

So the resistance fighters have moved the fighting underground.

They’ve carved a network of tunnels to move supplies, rescue wounded soldiers, communicate between besieged towns and even attack the foundations of regime buildings.

What helps is that Syria’s modern urban areas sit on layers of ancient structures. Aleppo has been continuously inhabited since the Copper Age — around 6000 B.C. — and Damascus is two millennia older than Julius Caesar.

Dig beneath the street, and there is always a possibility you’ll run into older buildings.

“There has never in the world been such a thick network of tunnels as there is in Syria,” Syrian academic Salim Harba told AFP in 2014. “It started in Homs in 2012, and the army has since discovered 500 of them. But I think there are twice as many.”