Image copyright Alamy

Archaeological discoveries are more likely to be found by technology than with a trowel and a torch, writes classical historian Mary Beard.

If you want a vivid glimpse of ancient Roman life, the best place to go - after the more famous Pompeii - is the town of Ostia, a 30-minute train ride from the centre of Rome, near the coast. It's one of my very favourite sites. Beautifully peaceful, surrounded by shady umbrella pines, and, quite unlike Pompeii, you often have it almost to yourself.

It wasn't so peaceful 2,000 years ago. From the end of the 1st Century AD, Ostia was one the two main ports of the city of Rome. It's where many of the supplies needed to keep the million or so inhabitants of the capital alive were hauled ashore. And it had then the seedy reputation that most big ports have even now. In the early 2nd Century, the satirist Juvenal (admittedly one of the grumpiest old men of the ancient world) bemoaned the kind of clientele you'd meet in an Ostian bar: "Thugs, thieves, runaway slaves, hangmen, coffin makers", and, not so common in a modern port, maybe, "eunuch priests".

Ostia was not buried by volcanic debris. But as, eventually, the city of Rome declined, and the imports dried up, the whole port crumbled and was gradually covered over by sand. That's largely been removed, and you can now wander into those once-disreputable bars, and peek into the commercial warehouses and the multi-storey apartment blocks. To be honest, it's not all quite as ancient as it seems. Some of what we now see belongs more to the 1930s than to the 130s, thanks to a huge restoration and re-roofing programme sponsored by Mussolini. But it's still a place that makes a very good show of taking you back in time.

It's also a place that has recently become a whole lot bigger. Archaeologists used to think that the town was built on only one side of the river Tiber, where it joined the sea. They have just announced, in the last few weeks, that it extended over both banks of the river, and was roughly half as big again as we imagined. We now know that across the Tiber, there was a large built-up area with its own impressive circuit of town walls, including several towers, plus a series of vast warehouses - three laid out around a central courtyard, and another constructed on rows of columns, which is the biggest of the lot at 140x110m (495x360ft), making it larger than your average British cathedral.

In historical terms, this makes a lot of sense. The commercial properties in old Ostia have always seemed rather bijou, as if designed to be the centres for the import of pearls, spices and other small volume/high profit products. Here at last are the storehouses for the staples - for some of the nearly five million sacks of wheat, or 20 million litres of oil that we reckon must have come into Rome this way each year.

But don't start planning a visit to these discoveries. They haven't actually been dug up. They are the result of sophisticated scanning techniques, which can reveal on a computer screen all kinds of structures buried under the ground. To the naked eye, they're invisible. If you were to go to the site itself, you wouldn't see any walls or warehouses at all, just a rather unprepossessing field.

Image copyright Thinkstock Image caption The amphitheatre at Ostia, near Rome

This is how many of the most important archaeological discoveries are now made. Gone are the days of Heinrich Schliemann, who in the 1870s went down into an underground tomb at Mycenae, found a number of lavish burials and claimed to have "gazed upon the face of Agamemnon". Gone too are the days when I first ventured into archaeology (not exactly on the Schliemann scale, I must admit) at a drab little Roman villa outside Shrewsbury. I can still remember the shivery thrill when I first dug up a frankly rather insignificant piece of real Roman pottery.

Image copyright SPL Image caption Aerial photography can reveal the history of a site

The archaeological wonders of today don't come from heroic subterranean exploration, still less from the efforts of teenagers with their spades and trowels in damp Shropshire fields. They are much more often "virtual".

Virtual archaeology is not new. There's a long history to the science of seeing below the surface of the soil, without actually excavating. It didn't take long after the invention of air transport for people to realise that you could see things from above the earth that you couldn't detect when you were on it. In particular, grass and crops grow slightly differently, and at slightly different rates, when the remains of walls or of banks and ditches lurk underneath. Even though you wouldn't notice them on the ground, the patterns are clearly visible from the air.

This was dramatically shown already in 1906 when a military reconnaissance balloon was used to take an aerial picture of Stonehenge and the surrounding land. And over the next 50 years or so, the alliance of archaeology and the air force turned up evidence for everything from bronze-age huts to medieval field systems. You could even plot the progress of the Roman legions in Britain as they moved inexorably north to thrash the natives, thanks to the distinctive outlines of temporary camps which still left their mark in the crops - at least to the well-trained eye, looking down from a Cessna.

Modern science goes a lot further, and is a lot more sophisticated, than this. Archaeologists now use radar, for example, to penetrate the earth's surface and detect what's underneath. They measure the different magnetic patterns in the soil produced by the different structures below ground. And they capture yet more precise images from the air, or even from satellites in space. The military connection now extends to the use of "unmanned aerial vehicles" - "drones" to you and me - for taking aerial photographs. High-powered computers process the data and, crucially, combine it to provide detailed plans of the walls and floors, the holes and the hypocausts, that lie hidden below the surface.

Image copyright Alamy Image caption Ruins at Carnuntum, Austria

Ostia is only one place where amazing things have been "seen". At the Roman town of Carnuntum, near modern Vienna, next to the visible remains of an amphitheatre, the clear traces of a gladiatorial training school have been (virtually) brought to light, complete with a large turreted entrance, a mini practice arena, and what appears to be a set of small rooms for the gladiatorial squad. The archaeologists call these "cells", but if, like me, you have a rather more domestic view of gladiatorial lifestyle, you might prefer to call them "bedsits". But whatever name you choose, this can be nothing other than a purpose-built gladiatorial school, and the only one ever found outside Rome itself.

More from the Magazine Image copyright Other The British Museum's 2013 show of artefacts from the Roman cities of Pompeii and Herculaneum, buried in ash during an explosive eruption of Mount Vesuvius, was a sell-out. But could even greater treasures - including lost works of classical literature - still lie underground? Unlocking the scrolls of Herculaneum

On the face of it, there is almost nothing not to like in this new wave of hi-tech archaeology. Even with the relatively expensive equipment, it is far cheaper than traditional digging (which requires small armies of human labour). It is much, much quicker. And if you decide you want a closer look, it tells you exactly where to put the spade in.

Most important of all, it leaves the archaeological remains where they are safest - under the ground. The truth is that we've probably got rather too many ruins in the world already, and certainly more than we can preserve as we would like to. Left exposed to the elements, ruins just get more and more ruined. That's the iron law of ruins. And it takes superhuman effort (and vast resources) to halt that natural process. Why add to our problems by excavating more of them?

All the same, I feel a bit conflicted. Part of me thrills to the magic of the technology, and to the sheer bravura of displaying the plans of lost buildings, even lost towns, at what seems like the touch of a few buttons, without ever putting a spade in the ground. Yet I do also feel a bit nostalgic for the old-fashioned ways. For something is lost, as well as gained, when we take away that direct, hands-on contact with history, and replace it by images on a computer screen. The old idea of "gazing on the face of Agamemnon" may have been corny (and if Schliemann ever actually said it, wrong - whatever face he gazed on, it certainly wasn't King Agamemnon's). But it did convey something of the buzz of getting really close to the distant past.

Image copyright Alamy Image caption The future of archaeology? Computer mapping in an Egyptian tomb

It was those encounters with the scrappy bits of pottery in an undistinguished Romano-British villa, and stepping over the threshold of a Roman apartment block at Ostia (rebuilt under Mussolini or not) that first set my heart (and my brain) aflutter - and turned me onto history. And the awful truth is that, though I know in my head we have more ruins than we can take care of, when I see those outlines of that cathedral-sized, columned warehouse in Ostia, my heart just itches to get out my spade and my trowel and go and actually dig it up.

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