Al-Khazneh (The Treasury), Petra
Nabataean masons carved an entire 40-metre, two-story temple facade downward into a living sandstone cliff, with no seams, no assembled blocks, and apparently no written plan that survives — so exactly how was the work sequenced and scaffolded from the top down, who was it really built for, and what still lies unexcavated beneath it?
1st century BCE - 1st century CE (Nabataean), commonly linked to the reign of Aretas IV · Petra, Ma'an Governorate, Jordan