How does a modern designer make his mark on an 18th-century garden? By digging a hole...

The landscape architect Kim Wilkie's recent commissions have often involved historic landscapes, but though he has great respect for their past stories, he's not a restorer. He doesn't do pastiche.

The proposals he puts in front of his clients seem to involve an impressive amount of earth-moving: a grass amphitheatre at Great Fosters in Surrey, nine elegant grass terraces at Heveningham Hall in Suffolk, and most recently a staggeringly beautiful earthwork, christened Orpheus, at Boughton House in Northamptonshire.

Boughton, the home of the tenth Duke of Buccleuch, is unusual in that the busy Victorians never meddled there. Nor did the landscaper, Lancelot "Capability" Brown, who surely would have swept away the broad, formal canals first laid out at the end of the 17th century by Ralph Montagu. Montagu had spent time as an ambassador at the French court and it was a Versailles vision of straight lines, sheets of water, symmetry and formality that he brought back to Northamptonshire, a French idea, realised by a Dutch engineer, Leonard van der Meulan.

The choice of a Dutchman wasn't surprising. The Dutch were good with water and at Boughton there was never quite enough. The river Ilse, wandering through the park, had to be persuaded rather carefully to become the Broadwater, lying in a great formal rectangle below the west front of the house and the canals either side of it.

Ralph's successor continued working on the formal landscape at Boughton until it stretched over 100 acres of the park, the lake even bigger, the avenues of trees even longer and more magnificent than they had been before. With the spoil left over from digging out the lake, he made a mound, like a pyramid with its top chopped off, a huge earthwork 60 yards square at the base, which fits neatly beside the Broadwater (the spoil didn't have to be carted far) with a canal in front of it and another canal, the Boat Reach, on its third side.

Then Boughton went to sleep. Through a series of clever marriages, the family found itself with so many houses, they didn't need their Northamptonshire place any more. And that's why, when the present Duke's father first started work on the park, scooping centuries of silt out of one of the canals, all that early work was still there, though often difficult to see. The powerful, sculpted silhouette of the mount was disguised by self-sown saplings grown into trees. An understorey of brambles and elder confused the picture even more. The canals had become bogs, their once straight edges undermined, overgrown, drifting into the long grass of the surrounding landscape.

I've gone into all this because it's an important part of how Orpheus happened and why it is as it is. The present Duke, who inherited two years ago, was determined to press on with the work of restoring the Boughton landscape, with a detailed map of 1729 as his guide. But the map, a superb bird's eye view of the park showing the house looking over its formal sheets of water, its blocks of woodland and formal rides, had a curious blank square in the space facing the mount. The Duke asked Kim Wilkie to come up with an idea; the most interesting gardens and landscapes never stay still.

"It came instantly," says Wilkie. Standing on top of the mount (cleared in 2007), looking back across the blank square at the house, he suggested the answer was to go down rather than up, making it an impression of the mount, inverted into the earth. To his eternal credit, given the practical difficulties of digging such a huge hole and so deep, the Duke said yes.

So Boughton now embraces and absorbs an earthwork that is unmistakeably of the 21st century but which acknowledges and speaks to all the much older things going on round it. It's a clever trick, and one, I think, that could only have been carried out by someone who understands the "ghosts of occupation", Wilkie's term for the spirit of a place. It's a subtle piece of work but at the same time extraordinarily bold, fitting into the wider scheme at Boughton. When you are with it, it demands your full attention, enclosing you. It's been beautifully made, the edges crisp, the banks smooth, the standard of work a tribute to Lance Goffort-Hall at Boughton who managed the whole project.

From the house, you are scarcely aware of this important new element in the landscape. As it is set down into the earth rather than raised above it, that's hardly surprising. As you approach down a broad grass walk, lined with belts of trees, it's the mount that dominates. Then closer by, you begin to distinguish the brim of Orpheus. Closer still, you are at its very edge, a huge 60 yard square carved out of the ground, steeply banked in grass, with a shallow turfed ramp spiralling down inside to bring you to the dark square of water at its base.

We all bring our own baggage to places, mind baggage I mean, not carrier bags, and I was unprepared for the calm that Orpheus inspired as I shambled slowly down the grass paths to its centre. The mount, so dominant from ground level, gradually disappears as you descend; because the air is so still you hear sound differently; you are slightly disorientated by the sloping lines of the ramps laid out against the straight line of the rim. And then, at the top again, you realise how well it works as a skyscape, a James Turrell in reverse, the dark, still water at the base reflecting the sky and its ever-changing cast of clouds.

Why Orpheus? The immediate link is the sense of descending into a different world, as Orpheus went down into Hades to bring back his beloved Eurydice. Other connections are less easy to pin down. Or talk about. Memory. Loss. A kind of resolution (if you are lucky) between the two. But I hate being told what I'm supposed to be feeling or thinking. Just go, taking an open mind.

From now until 1 May next year, the park and gardens at Boughton House, near Kettering, Northants, are open by special arrangement only. Call 01536 515731 to make an appointment. From 1 May until the end of August 2010, they will be open every afternoon (2-5pm), admission £3.