Elaborate 19th-century grotto uncovered in a Gwent garden

Tim Smit at Heligan in Cornwall showed how effective the "lost garden" tag could be when you want to draw in the punters. John Harris at Dewstow House near Chepstow, Gwent, has an even more extraordinary story to tell about the man who lived in his house in Victorian times and the garden he made there. What on earth was in Henry Oakley's mind when in the 1890s he commissioned the well-known firm of James Pulham and Son to burrow under the croquet lawn of his modest villa, with its long views over the Severn estuary? For his client, Pulham created an underground world of caverns and passages, grottoes and waterfalls, pools, stalactites and boulders, all spookily lit by concealed shafts of light coming through grilles let into the lawn above. Why did this unusual place so quickly and completely disappear from people's memories? And how did John Harris, the down-to-earth, practical farmer who now lives at Dewstow, get drawn into this weird fantasy world of Oakley's?

Lots of questions then. Fortunately there are answers to most of them. But what I can't get into focus is Henry Oakley himself: a bachelor, an only son, left with lots of money from his father's tannin business, buying in 1893 a handsome, but conventional Welsh villa with fine Wellingtonias lining the drive. And then turning the place into a real-life fantasy: dungeons and dragons on a grand scale. He lived in the place until he died in 1940. But nobody ever seemed to go there. It was never open to visitors. Only after Oakley's death did an illegitimate son, Stanley Naish, emerge from the shadows, employing John Harris's grandfather, William Harris, to manage the farm that then surrounded the house.

Pulham is much more of a known quantity, the third James in a family dynasty that peaked during the 1870s when Pulham gardens, ferneries, grottoes and rockeries were springing up all over England. The first James Pulham had already invented a special kind of fake rock, assembling masses of clinker, overburnt bricks, waste stone, slag, and then pouring a mixture of Portland cement over the lot. On its own that could look like a dog's dinner, but Pulham's success depended on his workers' ability to mimic the natural colours and textures of real rock. They used ochre, iron oxide, crushed charcoal, lime and chalk to colour the render and while the surface was still wet, worked it with brushes, combs, damp sacking, whatever they thought would give a natural looking finish.

Natural looking. That was the key. "Though completed in only a few months, the stone had the appearance of having braved the storms of a thousand winters," said the Gardener's Chronicle of the first rock garden ever made of Pulhamite, begun in 1838 for John Warner of Hoddesdon Hall in Hertfordshire. Often Pulham mixed the fake rock with real tufa, as he did at Highnam Court, Gloucestershire, begun in 1849 for Thomas Gambier Parry. By the time the firm began work at Dewstow they had already completed monumental schemes at Sandringham, where they built an extensive series of rocky outcrops around a lake and a boathouse roofed in Pulhamite. At Madresfield in Herefordshire they had made an ambitious rock garden on three levels. At Battersea Park in London they threw up a vast rockwork of boulders arranged by the lake to suggest a geological "fault"; it gave the gardeners an opportunity to plant an amazing rockery.

Rock work in conservatories then became the new rage and at roughly the same time as he was working at Dewstow, Pulham constructed an extraordinary rock garden at Merrow Grange, near Guildford, with walks winding through increasingly elaborate rockwork to an underground grotto with a glazed roof at ground level. Artificial stalactites concealed heating pipes and there was a sunken chamber lined with real tufa in which ferns could take hold. Merrow Grange was the first Pulham work I ever saw and it's the only thing that comes near the incredible layout at Dewstow with its glassed-over subterranean caverns, its stalactites and passage that eventually liberates you by a series of lily ponds at the bottom of the garden.

So that leaves the third player in this story: John Harris, whose grandfather worked as farm manager for Oakley's son, Stanley Naish. When Naish sold Dewstow in 1948, William Harris bought the farmland but not the house. That's when the garden seems to have been abandoned and the underground grottoes and passages filled in. The great tropical greenhouse where Oakley had grown his palms and orchids was turned into a Dutch barn (you can still see the elegant iron columns and brackets inside it). When the M4 was built, more spoil came up to Dewstow and much of the site was concreted over to make a more efficient farmyard.

By the late 1980s, John Harris was busy turning his farmland over to golf. The first course was hugely successful, so he built another and then, when the house came up for sale in 2000, the Harris family bought it and moved in. Like many local boys, John Harris knew the lower lily ponds of the six-acre garden, but had no idea what lay underneath the area closer to the house. Concrete slabs covered the skylights; stock grazed on grass over the subterranean tunnels. Nobody in the family or the area roundabout had ever heard of James Pulham.

But there were a few clues above ground – the entrance to the whole system and a collapsed roof in the yard where once a bullock fell through to Pulham's underground fernery. Harris never meant to excavate the garden, but having the men and machinery to hand, he started to pick away at the first blocked-up tunnel and then couldn't stop. "Basically, it was just curiosity," he says, shrugging. "And it was extraordinary, once we started digging away, to see how well all these structures had survived." Oakley left no plan and most of Pulham's records disappeared in a warehouse fire in the last war. "Nobody had any idea where the tunnels went or how they connected up, but I reckon the whole system must have been built at once because otherwise they wouldn't have been able to get the water flowing through."

Water, in various guises, adds a great deal to the mystery of Oakley's underground world, dripping through holes, drilled unseen in mossy rocks, swelling out into pools, with stepping stones across. "Beautifully put together," says Harris appreciatively. "The levels are exactly right." Carefully made grooves in the Pulhamite cement floors carry away any overflow. When Harris finally connected up the water again, the system worked perfectly.

As we are standing in the gloom underground, admiring some of Pulham's work, Harris describes the day he and his men were standing in the same place, facing a wall blocking off the passage ahead. Together they bashed down the top half, making just enough space for Harris to climb over it into the blackness and feel his way through the dark into the unknown. Eventually he worked his way right round to behind where they had started. "Then I crept up and tapped the guys on the shoulder. Didn't they jump."

Now, although he knows there's more to find, work has come to a halt and Harris has decided the place needs to earn its keep. It's wonderfully bizarre and is open every day (10am-4pm) from now until 18 October. Admission £6.

For more information call 01291 430444 or go to the website, dewstow.co.uk. Dewstow is one of 27 gardens covered in Helena Attlee's new book 'The Gardens of Wales' (Frances Lincoln £16.99)