In the early fifth century, rampaging Goths swept through Italy. Inviolate for 1,100 years, Rome was sacked by the hordes in 410 AD. St Augustine's apologia, the City of God, set the tone for Christians for the next 16 centuries.

But the Rome of that era came close to suffering a far worse calamity. A small metallic asteroid descended from the sky, making a hypervelocity impact in an Apennine valley just 60 miles east of the city. This bus-sized lump of cosmic detritus vaporised as it hit the ground. In doing so, it released energy equivalent to around 200 kilotonnes of TNT: around 15 times the power of the atomic bomb that levelled Hiroshima in 1945.

Pescara is on the Adriatic coast, located across the Italian peninsula from Rome. Housed there is the International Research School of Planetary Sciences, where staff and students study topics ranging from planetary geology to astrobiology. In 1999, a young impact cratering specialist from Sweden, Jens Ormö, arrived to take up a three-year position funded by the European Union.

Ormö, it happens, is keen on hill walking, and just inland from Pescara are some of the most spectacular mountains in the Apennines. He decided that some hiking in the area of the Sirente Massif was in order, and so he consulted a local guidebook. As he thumbed its pages, Ormö came across a photograph of something that amazed him. What he saw, labelled as a natural lake, was surely an impact crater.

An expedition to the site of the putative impact, on the Sirente plain, was hastily organised. Colleagues confirmed Ormö's initial suspicion. Here was an impact crater about 140 metres wide, previously unrecognised despite lying only a short distance from a busy road, and visible from miles away. It has appeared on maps for centuries, and in guidebooks for decades - but no one had recognised its significance.

Natural lakes are common in the area. But this one has a raised rim, now about two metres high, but originally rather thicker. This was produced by the asteroid throwing material out from the impact zone, as it crashed at a speed of around 20km per second, producing a huge explosion. Later filled with rainwater, the crater is now only a few metres deep, and occasionally dries up during hot summers. But it was more than 30 metres to the bottom when first formed. Centuries of weathering has eroded its bank and gradually filled it in.

Relatively modest craters like this are unusual, because small asteroids can only reach the ground intact if they are metallic, and thus strong enough to withstand the physical shock of slamming into the atmosphere at such speeds. The best guess at present is that the asteroid was about 10 metres across, and had a composition similar to nickel-iron meteorites. If it had been stony in composition, as most asteroids are, it would have shattered in flight and released all of its energy in a phenomenal explosion. This is what happened when a 50-metre rock blew up over Siberia in 1908, leaving no crater.The expectation of a metallic impactor is backed up by the identification of rust grains in the surrounding soil.

Confirmation of the impact origin comes from 17 smaller craters, typically 10 metres wide, scattered around the Sirente plain. These are due to fragments of the asteroid that separated in flight through the atmosphere. A magnetic survey shows that most are associated with anomalously high fields, indicating sub-surface metallic lumps.

Crater fields like this are not unusual. In central Australia, 120km south of Alice Springs, the Henbury craters were formed in a similar way. What is peculiar about the Sirente crater is where it occurred, and its youth. Dozens of ancient craters are known in northern Europe, geological stability allowing their long-term preservation. Two examples are the Ries and Steinheim basins in Germany. Many others are known in Scandinavia. But these are all huge, and millions of years old. There is a small, recently formed crater in Estonia, but the Sirente crater is of far greater interest: it was excavated around the time of the fall of the Roman Empire, and close to Rome itself.

The crater has been dated through radiocarbon analysis of a drill core cut down through the bank. The uppermost material, having been thrown out of the cavity, contains organic matter older than the impact. At the original ground level the radiocarbon ages minimise, and then deeper down the material is older again.

The data indicate that the crater was formed in about 412 AD, with an uncertainty of 40 years in either direction. Additional sampling may allow this spread to be reduced, but it is clear that the event occurred close to the fall of Rome: some time between 370 AD and 450 AD, when the city was again under attack, this time by the Vandals.

No matter what the trajectory of the asteroid entry, it would have been a phenomenal sight from Rome, and scarier still for those closer to ground zero. The fireball produced would have only lasted 10 seconds or so, but would have been brighter than the sun, and so visible even in daytime. The smoke trail left in the atmosphere would have been visible for some hours.

Another remarkable aspect of the event is that the main crater sits squarely in the middle of the Sirente plain, which is only about a mile long, and half that wide, being surrounded by mountainous terrain. It could be that this is just luck. Alternatively, the array of craters now identified might represent only a tiny fraction of the havoc wreaked, with many other impacts on the mountainsides having long since eroded or been hidden by tree growth.

Even considering simply the energy involved in forming the known crater, it is sobering to ponder what might have happened should the impact zone have been on the flat coastal plains nearer Rome, rather than in the mountains. Scaling from nuclear bomb tests indicates that a 200 kilotonne surface explosion would devastate an area of 100 square kilometres.

A frequently used aphorism says that Rome was not built in a day. That's true. But it did come awfully close to being destroyed in seconds.