Had seen this footage before on TV - was it Discovery or something. Long time ago.
This natural's tunnel engineering expert. They should make a 3D computer model in wire-frame and analyze that structure. How they managed to have sufficient oxygen and ventilation is related to the layout. I think the HILL structure on the surface may function as Air Scoop or Wind Scoop or air-intake, that it faced a constant natural wind direction and provided a funnel effect to bring draft inside. The tunnel's direction and angles must had also permitted air flow.