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A fan of square holes

A cluster of small quadrangular pits on Pompeii's northern wall, read as the firing signature of an ancient repeating dart-thrower.

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Two ballistic signatures on the northern wall of Pompeii, with a side-elevation schematic of the polybolos mechanism A two-panel plate. The upper panel shows a section of Pompeii's northern defensive wall drawn as ink outlines: six tufa ashlar blocks in two courses, joints visible. On a block in the upper right, a fan-shaped cluster of nine small quadrangular impact pits is rendered in rust, arranged along a shallow curve, packed close together. On a block in the lower left, a single large round crater is rendered in rust as a comparison: the signature of a spherical stone projectile from a ballista. Labels identify each: 'fan-shaped cluster of small quadrangular pits — read as polybolos' and 'circular crater — ballista stone'. A scale bar marked one metre runs below the wall. The lower panel is a side-elevation schematic of the polybolos itself: a wooden frame, a sliding mensa or carrier plank running front to back, a hopper magazine of stacked bolts above it, a flat-link chain running from the rear of the mensa over a small sprocket to a windlass with a crank handle at the rear, two torsion springs and curved arms at the front holding a drawstring, and a claw latch on the mensa gripping the drawstring. A single bolt is shown leaving the firing slot toward the left. Labels mark hopper, chain, mensa, claw, windlass. Pompeii · northern wall, between the Vesuvio and Ercolano gates · two ballistic signatures fan-shaped cluster of small quadrangular pits circular crater · ballista stone (for comparison) ≈ 1 m · approximate scale of the wall section Polybolos · side-elevation schematic, after Philo of Byzantium and the standard reconstruction torsion torsion sliding mensa claw hopper magazine notched feed pole drops one bolt per stroke bolt in flight chain windlass As the windlass turns, the mensa slides back; the chain pulls; the claw catches the drawstring; the feed pole drops a bolt; at the back of the stroke a fixed lug trips the claw and the bolt is released.
A wall, a fan of small square pits, a round crater, and the machine the pits would belong to.

In a paper published in Heritage on the 28th of February, Adriana Rossi, Silvia Bertacchi and Veronica Casadei argue that several clusters of marks on Pompeii's northern defensive wall — between the Vesuvio and Ercolano gates, on the section between Towers X and XI — were left by a polybolos.1 A polybolos is a chain-driven, magazine-fed dart-thrower; Philo of Byzantium described one in the third century BC. None has ever been found. Until now the polybolos was a passage in a treatise, a thing the ancient world had said was possible.

The argument is geometric. Where a ballista throws a stone, it leaves a round crater. Where a polybolos throws a dart, the team find — repeatedly, on the same wall section — fan-shaped clusters of small quadrangular pits, "arranged at short intervals along a curved line." The shape of a single mark would identify the projectile; the shape of the cluster identifies the firing rate. To produce a curve of square pits packed that close, one bolt after another must follow nearly the same trajectory, fast.

Lucius Cornelius Sulla besieged Pompeii in 89 BC, in the Social War, when the city joined the Italic allies against Rome. The marks have been on the wall for two thousand one hundred and fourteen years, waiting to be read as a fingerprint and not as wear. The reading needed photogrammetry, mesh modelling, and a 3rd-century-BC engineer to match the geometry to. The wall kept it.

  1. Adriana Rossi, Silvia Bertacchi and Veronica Casadei, "From Pompeii to Rhodes, from Survey to Sources: The Use of Polybolos," Heritage 9(3): 96 (28 February 2026), doi:10.3390/heritage9030096. Project context: SCORPiò-NIDI (PRIN22). The wall section, the gates and the comparison with ballista craters are drawn schematically: the cluster is shown as nine pits arranged along a shallow curve to convey the geometry the paper documents, not the exact count or spacing of any one cluster, which the paper measures in three case studies whose figures are paywalled. The mechanism schematic follows Philo's description as reconstructed in the secondary literature: hopper above the mensa, a flat-link chain — the earliest known application of a chain drive — connecting the rear of the mensa over a sprocket to a windlass, and a claw latch that the back of the stroke trips. Reporting that situated this discovery for me: Lost Ancient Automatic Weapon Fired at Pompeii? (Archaeology Magazine, 13 April 2026), and the project's own English summary at the Il Mattino newsroom. ↩

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