Credit: AMERICAN GEOPHYSICAL UNION

Chance physical phenomena can intersect with human civilization in unexpected ways. One such phenomenon is a putative meteorite impact in Italy's Sirente region dated to around AD 400: it has been speculated that the fiery arc traced by the meteorite fragments in the sky was instrumental in triggering a chain of events that eventually led to Christianity displacing pagan beliefs in the Roman Empire.

The primary evidence for an impact in this region is the presence of an approximately 100-metre-wide sag or depression, accompanied by other smaller sags. The morphological attributes and distribution of these features have been considered consistent with crater formation due to a meteorite shower. However, this interpretation is by no means unique. Several features typical of impacts, such as shocked minerals and high concentrations of certain metals, have not been found, and the craters have alternatively been proposed to be mud volcanoes, pits dug by humans or sink holes, that is pit-like features that commonly form when water dissolves lime.

Resolution of the craters' origins requires detailed information about the subsurface structure of the sags, which is now presented by Speranza and colleagues (J. Geophys. Res. doi:10.1029/2008JB005759; 2009). According to the team, the electrical and magnetic properties of the area's sediments and rocks show unambiguously that none of the crater-like structures were formed by an impact. Furthermore, geological and geochemical data — such as the absence of methane or carbon dioxide reservoirs at depth — rule out a mud volcano origin.

The survey shows that the sags are underlain by a thin sedimentary package that rests on a series of ridges and valleys cut into a limestone substrate. Sediment-filled depressions in the subsurface ridges, indicative of sink holes, underlie many of the smaller sags. The researchers conclude that water seeping through the sediments led to the formation of sinkholes at depth, which ultimately caused the surface to cave in.

The main crater-like feature is now occupied by a lake. Layers of sediments within and underlying this lake show no sign of being disturbed and are more or less horizontal, which is inconsistent with an impact. The properties of these sediments and those surrounding this sag are rather similar and it is therefore unlikely that sediments in the structure represent impact crater fill. Moreover, the magnetic signature of the material at the bottom of the main sag is quite the opposite of what would have been expected for a buried meteorite.

Depressions with a size similar to the main Sirente sag are also found in nearby hill ranges; the researchers have previously proposed that these are man-made. The region's economy has depended on sheep rearing for thousands of years: water flowed from springs and accumulated in these sags, which served as a drinking trough. Speranza and colleagues suggest that the Sirente crater served a similar purpose and is in fact a water reservoir made by humans.

The Sirente sags appear to have been emplaced under far calmer circumstances than a meteoritic impact. Their birth is unlikely to have swung Roman history, but probably helped satisfy many a thirsty lamb.