That's one mystery less. Ventilation at the ends of the tunnels is another mystery. It was heavy labor knocking out those cavernous spaces. All I can think of

That's one mystery less. Ventilation at the ends of the tunnels is another mystery. It was heavy labor knocking out those cavernous spaces. All I can think of is they just abandoned work for a while when the air got too bad. (Piped-in air seems too unlikely.) Lots of the Valley of Kings tombs were unfinished when used, so it might just have been slow going. Heavy stonecutting out in the open involved building a fire against a stone face and then quenching it to fracture and weaken the rock. But that wouldn't work in those tunnels. The ancients worldwide cut through fantastic amounts of stone, often in close quarters. Ancient Celts invented hard-rock mining, cutting deep into mountains. They all solved it, somehow, likely differently in different places. But details matter.