Wael Soulaiman picks his way carefully around the edge of the three strips of hardened dirt, each about the length of a cricket pitch and only a little wider.

To the ignorant observer, they do not look like much. In fact, there is something incongruous about this entire corner of segregated scrubland in the deserted suburb of Eastern Ghouta, five miles from the centre of Damascus.

Although the strips of earth are clearly man-made, they have an abandoned feel, defaced by small craters and the scrapes of a digger, with little to denote their true purpose.

It is only when Soulaiman begins to speak that their awful secret becomes clear.

“There are 400 in this one,” he says. “Bodies – women and children. 400 hundred women and children in that one, too. Over there” – he points to the final strip – “it’s the men. About 600, we think.”

The 42-year-old runs a hand through his hair and looks around him at the concrete innards of the shattered apartment blocks that surround the site.

He is sombre but matter of fact – remarkable given that the multitude of corpses lying beneath this nondescript earth number four of his close family, the youngest just eighteen months old.

“I was responsible for bringing them here,” he says. “There was nothing more difficult.”

He goes on to explain how they arranged the remains: body after body packed closely alongside each other; a membrane of soil little thicker than the length of a school ruler, then another layer, and another, and on.

If there is little to mark out this walled-off acre as a mass grave, it is because that is how Bashar al-Assad wanted it.

In August of 2013, the dictator was on the verge of losing control, with the rebel uprising stretching his security forces to breaking point.

He had already become infamous for his brutal use of artillery on civilian areas.

But as 2013 dragged on, international observers had warned that he was liable to go even further, resorting to his fearsome stockpile of illegal chemical weapons to regain the upper hand.

Indeed, Barack Obama had stated unequivocally that to do so would cross “a red line”.

But in the early hours of Aug 21, Assad gambled everything on the West’s indifference.

First his soldiers launched a conventional artillery bombardment.

Then came something worse. Abo Anan, a woolworker, stepped out onto his balcony to survey the damage.

“I saw three missiles come down in the neighbourhood, but instead of exploding, it was a thud,” he recalled. “Then they started hissing. We didn’t understand it but I thought it was bad. Later, all my family started vomiting.”

It was sarin, one of the most deadly nerve agents ever to be invented.

Ultimately, the gas released into the densely populated neighbourhood that morning killed around 1,500 in the immediate aftermath, experts believe, with countless more dying later.

Heavier than air, the gas found its way into the cellars where families were sheltering, its passage eased by the doors and windows broken by the earlier barrage.

Later, video would emerge of mothers screaming over their babies who were foaming at the mouth, before beginning to spasm themselves.

It was the most brazen and deadly chemical weapons attack since Saddam Hussein launched mustard gas at the Kurdish population of Halabja a quarter of a century earlier.

“I helped move some of the bodies but then I couldn’t lift them any more because I became intoxicated. That was the scariest part.”