Things look different underwater. Firstly, there is the optical deformation which magnifies anything seen, but apart from optimistic fishermen, the rest of us compensate for the effect. Then there is the magic of the new and strange world which is part of the pleasure of diving and snorkeling. Looking at an archaeological site underwater has a special fascination which even the most knowledgeable and blas of terrestial visitors cannot escape.

Now there are two sites close to Rome where the amateur is taken round what until recently was open only to specialists. One is Pyrgi, the Etruscan and Roman port of Cerveteri 60 km up Via Aurelia whose coast has been eroded by at least 300 m. The other is the Roman town of Baia, which sank slowly into the Bay of Naples a millennium and a half ago. For the moment you have to don wet suit, goggles and a scuba cylinder to see them, but both will soon have glass-bottomed boats available and Pyrgi is shallow enough for visitors to be able to swim on the surface with just a snorkel.

Underwater tours have become possible only with the advent of full face masks, which leave the divers mouth free. The archaeologist/guide has a microphone in the mask and the visitors clip a receiver onto their mask straps to cover their ears. Even when visitors go round the site safely ensconced in a glass-bottomed boat, the guide is there underwater with microphone and camera pointing to the objects close up and describing them. Fish swim past the lens and the guides breathy bubbles punctuate description.

And so the tour begins.

According to ancient sources, at its height Etruscan Cerveteri was as big as Athens. Locals proudly repeat the fact today. Cerveteri has one of the biggest as well as finest necropolises so it is not surprising that its port was also impressive. The ancient name of the port was Pyrgi. Today it is called S. Severa, a fierce castle and keep on the beach half a mile from the modern path of Via Aurelia.

On land there is the mediaeval structure; little houses crouch around the keep and the walls. There is a baroque chapel and the whole sits on impressively visible, finely dressed blocks of the Etruscan and Roman period. On the coast to the east is the temple area where the Pyrgi gold tablets, one of the few pieces of bilingual text with Etruscan, were found in 1967. The port area spreads out in a 300-m quadrant from the temples to the entrance channel in front of the keep, clearly visible underwater if you climb to the top of the tower.

Visitors are taken through the castle into a garden on the beach where they prepare for the dive. The local Gruppo Archeologico Cerite led by the S. Severa museum curator Flavio Enei and archaeologist Giuseppe Fort (who speaks English) have produced a team of divers capable of using the special equipment necessary as well as knowing the site stone by stone. Enei himself has a huge enthusiasm for his work and the site, which comes across despite all the bubbles and microphones and earphones.

All archaeology is hypothetical until a better theory is put forward, but whereas in old sites most theories have been around for decades, here work has only just begun and even the one-off visitor is part of the discovery process.

Enei has discovered what seems to be a quayside with columns probably laid out in an arcade on top; there is a section of an arch and several wells, some of which were discovered only last year. They were used either as rubbish tips or, suggests Enei, they might have been part of some metal-processing plant, as we found some iron ore which had been partially worked. The metalworking hypothesis is confirmed by a load of iron ore which they also found last year, but now weve discovered the timbers of the ship below the pile of ore, says Enei with great excitement. One possibility is that this was a cargo of ore from Elba which was due to be worked in Cerveteri or Pyrgi but which sank when it arrived and was never salvaged.

Between the masonry and the possible wreck there is a quantity of pottery which reveals how much was going on in Pyrgi 2,000 years ago: amphorae handles and pieces of pots, some encrusted with algae and weed, others just uncovered from the sand and so clean-looking as if they had just been broken. On dry land again, you can look at the two small museums which have objects from the site.

Further afield but in clearer water and without the destruction of erosion is Baia, a once-fine resort town in the Phlegrean fields close to the big commercial port of Puteoli (modern Pozzuoli). Slow earthquakes from the fourth century onwards gently sank Baia and it was not rediscovered until new port installations were built in the 1920s. Serious archaeology began after world war two and now tourists can visit the site with its thermal baths and patrician mosaics.

Pyrgi. Gruppo Archeologico Cerite. 14 and 20 July, 4, 10 and 25 Aug, 7 and 15 Sept 2002. For the whole of July 2002 there is an underwater survey. Dr Flavio Enei, S. Severa Museum, 0766570077, 0766571727. Prof. Giuseppe Fort, tel. 335/6498198. www.torreflaviadiving.it, www.gatc.it. Baia. Guided tours daily, winter and summer, weather permitting. 35 including tank. Tel. 0815248109, fax 0815249850, info@baiasommersa, www.baiasommersa.it.