Think of St Paul's Cathedral and the amount of Portland stone that went into its construction, then try to imagine a corresponding hole in the ground and you might begin to visualise somewhere like Tout Quarry.
The quarry, just off the coast of Dorset, appears on tourist maps as a sculpture park. It is also a Site of Special Scientific Interest - but neither description comes within light years of doing justice to this place. I think it's the most inspiring, and surprising, 40 acres of ex-industrial land you can find anywhere in Britain.
You don't believe me? You will - but first you have to get there and Tout, for all its fame in sculpture and stone-carving circles, is well hidden. It's on the Isle of Portland, itself a place apart. As Thomas Hardy wrote, arriving here was like "leaving one world and entering another".
As you approach by road from Weymouth, the "isle" (which is really a peninsula) rears up above you, dwarfing the terraces of Fortuneswell on its slopes. A series of hairpin bends takes you up above the town and out on to a plateau hundreds of feet above the English Channel.
The views are fantastic: Chesil Beach to the west, Lulworth to the east, and north towards the world that, for the past 600 years, has been chipping away at this place to build its cities. South, across Portland itself, is the evidence - a cratered landscape that gave Wren, Hawksmoor and scores of other architects their favourite stone.
Tout is the highest point of the peninsula - the name, pronounced "towt", is local dialect for "lookout point". But entering the quarry through a tiny V-shaped gap in a stonewall, you are once again plunged into a very different world - a labyrinth of deep gullies rich in vegetation overshadowed by vast stacks of discarded rock. When you've satisfied your childlike urge to scramble over boulders, you can start spotting artworks. There are said to be more than 50 here.
Some, such as Antony Gormley's "Still Falling", a lifesize figure frozen in mid-dive, are carved directly into the rock walls. Others, such as Christine Fox's "Serpent Steps", follow and accentuate the landscape, a path which becomes the tail of some colossal prehistoric dragon whose spine follows the curvature of the heights above the quarry. Or how about a perfect, classical fireplace carved by Timothy Shutter into the limestone wall of a grotto - which, like all the best surrealist jokes, is more than a joke, all the stranger for being blackened by the soot of the fires made in its hearth.
As you are repeatedly astonished and delighted by these finds - and sometimes you have to uncover them behind veils of trailing ivy - you should also keep an eye open for rare orchids, birds and butterflies. Tout's SSSI status is well deserved - it's a naturalist's paradise.
All summer, the quarry resounds to the ringing of chisels against limestone. You can navigate your way through the maze and towards the workshop area simply by following this eerily musical sound. Here, a group of people are carving and learning to understand the qualities of this special stone. The summer courses are open to all ages and attract people from around the world. Where else would they get to work with such stone, in such a setting, learning from the skills of sculptors, quarrymen and stonemasons who have been working the material most of their lives?
At other times, you'll find school children and their teachers, art college groups, or undergraduates from the University of Brighton taking a BA elective landscape and environment course developed here. You're aware, by now, that Tout is a very special place - and are probably wondering how this rare collision of art, nature, industry and education came about. It was not by chance.
Tout had been left to nature for almost a century when, in 1983, Britain's first open-air sculpture symposium was held here. Artists were invited to work in the quarry, using its stone and landscape as their raw materials. The event was a huge success, and everyone involved knew it had to be continued. The Portland Sculpture amp; Quarry Trust was formed to develop an educational programme which would go beyond sculpture to involve geologists, botanists, landscape architects, film-makers and photographers.
Since then, Tout Quarry's owner, Hanson, Bath and Portland, has awarded a new 30-year lease allowing the project to continue. And the trust has gained the use of an old drill hall - with space for studios, exhibitions and a long-proposed education centre - through a partnership with Albion Stone Quarries and Crown Estate.
This could also see the project extending into two further disused quarries. In one of these, Independent Quarry, the quarrying has left a vast tiered limestone pit which could make an extraordinary auditorium. An artist is already working in the quarry on ideas. Plans for Jordan Quarry include the use of its remaining limestone for works of art: one cubic metre a year, which would give it a new lease of artistic life for a good 100 years.
Again, I'm made aware of the special qualities of this place and the people who work here. They think in decades, while I'm thinking in days. But what is 100 years on an island whose stone has been 135 million years in the making? I sense that the project germinated in Tout Quarry will soon have a much higher profile.
Portland Sculpture Trust, 221 Archway Road, London N6 5BN. Tel: 020 8341 6742