Ħal Saflieni is the oldest large example of architecture built not by adding but by carving. While the temples above ground on Malta rise by stacking stone, here the opposite is done: the soft globigerina limestone floor is dug away and the space taken down through three levels, hollowed chamber by chamber. The space is not constructed; it is removed from the mass.

What is striking is that these underground voids imitate the formal language of the stone temples above. Ceilings are cut to look like corbelled vaults, walls carry carved post-and-lintel orders and convex coursing — yet here nothing is load-bearing; every form is a memory of building incised into solid rock. The way people raised structures above ground is retold below ground by carving those structures in negative.

The lowest level's 'Oracle Room' uses the acoustics of a cavity deliberately: a male voice at a particular frequency resonates through the whole hypogeum; the room is a vessel for sound cut into stone. The structure is also a necropolis — it held thousands of burials. Ħal Saflieni shows, already in prehistory, the second of the two ways to make enclosed space — building by subtraction — a line that will run from the corbelled vault of Newgrange to the rock façade of Petra and the monolithic churches of Lalibela.