Dolceacqua is famous for its Roman footbridge, painted by Claude Monet, who described it as "a jewel of lightness". The town, a few kilometers inland from Ventimiglia, is firmly on the international tour bus routes and deals daily throughout the season with visitors of all nationalities, who wander round its intricate narrow streets ("carruggi") up to the Dorian castle then have pizza in the piazza.

But there's another reason for coming to Dolceacqua, one far less well known yet even more not-to-be-missed. Hidden away in the middle of the Old Town is the "3-D Visionarium", a globally unique concept that leaves visitors gasping with wonder. I could describe it as a slide show, but that wouldn't begin to do it justice. So let me tell you about the experience.

From the far end of the piazza in the Old Town, enter the carruggi, turn right and walk along and up, past posters of the Irian Jaya people of Papua New Guinea and under a replica of a forest house. A short way along is the Visionarium, an unimposing wooden door with a list of opening times. If you arrive during one of these you may hear music and commentary coming from inside, and the door may be open or closed. Knock if necessary.

The proprietor is a reticent, dour Italian photographer who speaks enough French and English to cope with visitors. A number of shows are available, but only one has an English soundtrack (also available in Italian, French and German), so make sure you ask for the right one. Just say "the Nervia Valley". It'll cost you about €3-4 each and he might not charge you at all if you've been before. Which many, many people have. It's that kind of show.

After paying you're issued with 3-D spectacles and you go inside, to a room with church pew seating able to hold 30 or so people. The room is basically a cellar, with stone walls and a vaulted ceiling, but it slopes like a proper cinema up from the screen at the front. Behind you are the projectors; not just one but maybe a dozen (I never thought to count them).

The subject of this show is the Nervia Valley, on which Dolceacqua sits. Now this may not sound like a promising use of 20 minutes, but you're in for a surprise. Several, in fact. First there's an introduction taking a couple of minutes, then the main show is divided into five or six parts, based loosely on the seasons. And it's here where the 3-D experience starts.

It's impossible to describe in words the impact of the photographs, some of which are in fact short video clips, I think - it makes little difference because the overall sensory experience is so overwhelming. From behind you is the soft clack-clack of the slide projectors; all around you the commentary and the music - Mozart, Tchaikovsky, Strauss, Vangelis and Enya, while the slides burst out of the screen at you. Even the domed ceiling is part of the show, at times becoming a star-filled backdrop. Scent generators fill the room with lavender and other floral scents, a real bell somewhere in the room chimes when a church is featured, and at one point a real rushing stream appears under the screen.

The photographs themselves are stunning, featuring the Ligurian scenery; mountains, rivers, Alpine meadows, ruined churches, and my particular favourite, the caves at Pigna, only reachable by serious cavers yet photographed in 3-D with professional lighting, at times making you feel you're standing up to your knees in an underground stream with a rope attaching you to the next man.

Once the show finishes there's one more treat. In a side room are two large models hand-carved from olive wood. One is of the castle; the other of the town itself. Both made by the same man, who devised the Visionarium and takes your payments, rarely revealing a flicker of emotion.

On leaving, everyone feels the same; a sense of awe, of being privileged to have experienced a true master-work in however a humble and unlikely setting. And a desire to return.

So, whether you're a visitor or a Riviera resident, go to the 3-D Visionarium. The bridge will be here for centuries but one day the Visionarium will be gone, swept away by "progress" or for want of anyone else to carry it on. The world will have lost a genuine treasure.