(BBC) -- In the middle of the Iraqi desert, just north of the city of Mosul, is a pile of limestone blocks. Even at first glance, they're unusual -- each half a metre (1.6 feet) across, with a square shape and one side decorated with enigmatic symbols. Collectively they look like the remains of a giant pyramid.

But this was not their purpose.

An inscription, meticulously engraved into one side, explains. It's a message from a man called Sennacherib, an ancient Assyrian ruler who lived more than 2,700 years ago.

"[I am] Sennacherib, king of the world, king of Assyria. For a long distance from the river Hazur to the meadows of Nineveh I caused a canal to be dug. Over deep-cut ravines I constructed an aqueduct of white limestone. These waters I caused to flow over it."

The ruins at Jerwan are among the only physical remains of this immense project. The artificial channel was hand-carved into the desert landscape with iron pickaxes -- and sent water burbling 150km (93 miles) across its parched surface, diverting it from the mountains to the ancient city of Nineveh. Sennacherib had redirected the course of an entire river towards the kingdom's capital city and his home.

In particular, the antique rubble is what is left of the aqueduct's piece de resistance, a 280m-long (922ft-long) water-bridge erected to guide the river across a steep-sided valley. It was made from an estimated 443,520 blocks, each weighing around 250kg (551lbs), with soaring corbelled arches running along its length -- bringing the grandeur of a Victorian viaduct to an otherwise empty desert, in 690BC.

Back at Nineveh, Sennacherib was careful not to squander the watery possibilities that this unprecedented feat of hydraulic engineering provided. Initially, the goal was to use the new bounty to quench a belt of freshly planted orchards and grain fields above the city, along with vines, "every type of fruit", "products of every mountain", spices, and olive trees. But two generations later, his grandson had another idea.

Ashurbanipal was a brutal warrior, enthusiastic lion-hunter, and the ruler of the Assyrian empire when it was at its most extensive -- stretching from the snow-capped peaks of modern-day Iran's Zagros mountains to the sparkling waters of the eastern Mediterranean. But when he wasn't spearing large carnivores or conquering new lands, he enjoyed a spot of gardening.

During Ashurbanipal's 38-year-reign, he transformed the space around the city's many royal palaces into a series of enchanting and elaborate gardens, the most impressive the world had ever seen.

In one relief sculpture from the era, Ashurbanipal and his wife are reclining on a kind of high wooden sofa, sipping some refreshments amid a lush garden oasis. A large grape vine provides shade above them, and they're nestled among date palms and pine trees -- surrounded on all sides by vegetation.

As they lie there, the couple are being serenaded by musicians and fanned by servants, with Ashurbanipal 's fabulous long, braided beard fluttering gently in the breeze -- or at least, one imagines it might be. It's an archetypal garden paradise -- that is, except for the disembodied head of an enemy, which is hanging from a nearby tree. A hint that this was a very different time, despite the mutual appreciation for greenery.

Elsewhere in the gardens, which are enclosed within heavily fortified walls, there are cedar, juniper, cypress, ebony, myrrh, pear, pomegranate and fig trees. Tame lions lounge in the shade. Birds flit from palm to palm. Parrots survey the scene from the canopy above. In another relief, there's a mountain looming in the background, with rows of plants arranged in terraces on its surface and a lake below. Others show large trees growing on tiered structures held up by Corinthian columns.

But as abruptly as the gardens at Nineveh appeared, they vanished -- outliving Ashurbanipal by just 19 years. The oasis was utterly forgotten, only rediscovered millennia later when the ancient palace was excavated in 1854. The archaeologist who unearthed them also found the remains of the aqueduct at Jerwan -- though he initially mistook it for a giant causeway.

How can a garden that was so ambitious, have been forgotten so comprehensively? And what would it take for a garden to survive hundreds, or even thousands of years?

Whimsy and ancient yews

In a small corner of Cumbria, behind a modest grey limestone wall, lies a pocket of another world. Hundreds of strange shapes scatter the landscape -- a vast velvet-green umbrella, a towering top-hat, a sprawling judge's wig, whole herds of giant pyramids. There are balls, cylinders, cones, pedestals, and precarious tiered spires, each as tall as a house. The space is broken up by endless rows of looming emerald walls, each with its own cut-out arches -- together these form broken tunnels that stretch into the distance.

This storybook backdrop -- which is entirely constructed from trees and hedges -- is just about as alien as it gets. But there is one person it would be entirely familiar to: a Frenchman named Guillaume Beaumont, who designed it. The thing is, he died hundreds of years ago, in 1727.

Beaumont first planned out the Levens Hall gardens in the 1690s, and they were painstakingly planted and sculpted over the following two decades. Today, more than 332 years later, they're still there -- not just a faint echo of them, but the real deal, containing many of the exact same yew, box and beech topiaries that were planted in this long-vanished era. When the plants first sank their roots into Westmoreland soil, the dodo had only been extinct for a decade, there were only around 603 million people on the planet, and piracy was considered to be in its "golden age".

"How many successive generations have strolled pleasantly along those same long straight walks of gravel and sweeps of lawn," pondered George Frederick Weston in a history of the gardens in 1869, "...and have looked with pleasure on those fantastically-shaped yews and box trees and hollies, -- those same high walls of smoothcut beech, and those same stiff, box-bordered beds."

Weston, the vicar of a village church about 25 miles (40km) to the north, was fascinated by the gardens' permanence -- particularly the idea that he could see exactly what others had hundreds of years before, and feel what they felt.

Of course, surviving this long has not been easy. There have been wars. There have been changes of fashion. There have been financial hardships. And yet, through all this, the gardens have clung on.

Fashion and finances

One major factor in the gardens' survival is money.

The gardens at Levens Hall were commissioned by the English courtier and politician James Grahme, who was the keeper of the Privy purse -- the treasurer -- for King James II. "He was quite a wealthy man, and it was laid out in the fashion of the time," says Chris Crowder, the head gardener at Levens Hall -- one of only 10 in its long history.

In the 17th Century, this meant topiary. Fanciful hedges and trees like cartoon clouds were popping up in gardens across Europe, including a number of royal residences, from Hampton Court to the Palace of Versailles, and many of the most extravagant houses.

"And then by the early 1700s, everybody with wealth was ripping them out and chasing the next thing," says Crowder.

This next thing was the brainchild of the landscape architect Capability Brown, who remains a legendary figure in the gardening world to this day. His speciality was faking bucolic country scenes, with rolling pastures and scatterings of artfully-placed trees, lakes and staged ruins.

At the Chatsworth estate in Derbyshire, Brown created 1,000 acres (1.6sq miles or 4sq km) of parkland, contained within a 15km (9 mile) drystone wall. To achieve the effect, entire oak trees were dug up and dragged into more aesthetic positions.

"It was such a huge fashion that hundreds and thousands of earlier gardens were swept away -- people just wanted to keep up with the Joneses," says Linden Groves, head of operations and strategy at The Gardens Trust. "So in this country, it's quite hard to find surviving gardens from before the 18th Century," she says.

But while everyone else was tearing out their topiary, at Levens Hall it remained. One reason for this is that the Grahme family was so moneyed, they owned many houses -- so there was little incentive to keep their obscure little Cumbrian mansion up to date with the current trends. "It was a family backwater -- they were doing fashionable things elsewhere," says Crowder, explaining that it was a convenient place to stash poorer and relatively powerless female relations, such as aged aunts.

Then a stroke of male misfortune meant the female dominion over Levens continued on to the next generation. Though Grahme originally had three sons who could inherit the estate, each met an untimely end -- and it was eventually passed to his eldest daughter, Catherine, instead. By then, the women in the family had a deep affection for the quirky topiary gardens, says Crowder, and wouldn't allow them to be changed.

"There's definitely that feeling at Levens that it was because it was always lived in by the females for a long period [that it survived]," says Crowder. The final stroke of luck was a renewed appreciation of the old in the early 19th Century -- even antiques weren't valuable until then -- which meant that suddenly historic gardens were in-demand.

For Pam Smith, senior national consultant for gardens and parklands at the National Trust, these sudden fads are a bit jarring. "When I think about all the work we put into making decisions on how we assess, conserve and celebrate significant layers it can feel like quite a contrast with some of the more personal and ambitious decisions made by the historic families," she says.

Smith gives the example of Kedleston Hall in Derbyshire, where a new landscape garden was created in the 1720s. "It was a large, terraced garden ascending up the hill from the south of the hall," she says. "It must have been a dramatic and impressive sight." But when it was inherited by the next generation, the garden was considered off-trend and removed. "This amazing garden lasted only 40 years," says Smith.

At Levens, the gardens' survival -- and with most of their original features intact -- is almost miraculous. "The gardens weren't particularly great or different at the time," says Crowder. "But where all these others have changed completely, or some cases they are restorations, Levens has ridden through the years actually being gardened," he says.

Crowder points out that several-hundred-year-old trees are not exactly unusual -- though they are becoming rarer every day. It's the fact that the ones at Levens have been watered, pruned, preened, and carefully tended for centuries, that makes them special. "This garden has been crafted yearly, they [the topiary trees] have had people's hands across them, intervening every year," he says.

Derelict sheds and happy shrubs

So what happens when these carefully contrived creations are abandoned -- how long can they survive?

When Tim Smit and John Willis first came across The Lost Gardens of Heligan in 1990, the land belonged to brambles -- a great, twisting mass of spiky tendrils, each groping out to claim any buildings or open space within their vicinity. Trees had sprouted up from the glasshouse. Ferns had emerged in great Jurassic clumps. The local wildlife was tangibly thrilled -- nature had claimed the garden back for itself.

But hidden beneath this wild veneer, there was a nearly three-century-old garden. "Initially, it would have been all about the bare essentials, which are providing food for your household," says Alasdair Moore, the head of gardens and estate. "So there's a productive garden, which is now two acres and again, that developed over years and grew as horticultural skills and technology developed," he says.

The formal gardens were initiated by a local mining baron in the late 18th Century, who planted conifers and planned the landscaping. Over the coming decades, his family added a walled garden, glasshouse, "melon yard" (a flattened greenhouse for melons), and pineapple pit -- an ingenious method of growing the fruit in the chilly UK temperatures, involving trenches filled with manure that warms them up as it decomposes.

For generations, the Tremaynes kept the gardens well-manicured, occasionally adding new features as they came into fashion. Then after World War One, this became tricky.

"The main big, big shift -- but this is this is very, very common throughout the British Isles -- is the massive social changes," says Moore. Among other things, the transition from agricultural wealth to that generated by industry meant that many aristocratic families could no longer afford their country pads.

But for Heligan in particular, there was also tragedy. "Thirteen of the gardeners and estate workers went off to fight in the war and only four returned," says Moore, who explains that it had a profound effect on the skills of the garden workforce after World War One, and on the atmosphere on the estate in general. "Jack Tremayne [the last squire] talked a lot about ghosts -- the place being full of ghosts and not really wanting to be here anymore," says Moore.

From there, things continued to decline. The house was rented out, and the gardens were soon forgotten. They became progressively more wild, until Willis -- a descendent of the Tremayne family -- stumbled across some hints of their former grandeur on the estate one day in 1990.

"There were great billowing clouds of brambles. The laurel, which had been used as a hedging plant, had completely taken over. There were hundreds of self-set trees that had grown up. It really was 'take a machete and try and find a way in'," says Moore. All of the garden structures had rotted, he says, except for the walls -- some of which had fallen apart regardless.

The garden was in total post-apocalyptic chaos, though in some ways remarkably intact. When the team forced their way into the head gardener's office, Moore says there was still a kettle above the fireplace and a bucket full of coal that had coalesced -- possibly the only reason it hadn't completely disintegrated. "[There were also] pruning scissors, hanging on a hook in a glass house that had completely fallen apart," he says. It was almost as though the estate team had just walked off one day and never come back.

And in the midst of this wilderness, there were other buried signs of what had once been. Many of the original features were still there -- the pathways, though overgrown, were still discernible, and previously-contained patches of camellias and rhododendrons that were first planted in the early 19th Century were enjoying their newfound freedom.

"So the bones of the garden in terms of the planting -- woody plants, and large trees and shrubs were fine," says Moore. He wonders if some of the more shallow-rooted vegetation only survived the 1976 heatwave and drought -- during which some parts of the UK had no rain for months -- because the larger plants had grown up so much, locking in moisture and providing shade.

After discerning the garden's previous layout, an army of volunteers set to work clearing the plants that had self-seeded or spread too far beyond their usual remit. The buildings that could be salvaged were repaired, and today The Lost Gardens of Heligan is among the region's most popular attractions.

In Moore's view, while beauty and structure are important in any garden, the most important factor in their survival is the availability of people to look after them. "It's all about people -- not just to do the work, but people to care, and to be engaged. Gardens are driven by individuals who feel the need to express themselves through the medium of a garden," he says, laughing.

Venerable flowers and ancient trees

And while the jury's out on whether resurrected gardens count as authentically old, having ancient plants certainly helps.

One example is the Amazon rainforest. The latest evidence suggests that when Christopher Columbus set foot on the Americas in 1492, the Amazon wasn't quite as pristine or as natural as it was once believed. Instead it was shaped by a thriving indigenous community, whose cities were rapidly wiped out by European colonisers and their diseases. These abandoned settlements were then promptly swallowed up by the forest.

But tantalisingly, though it's been half a millennium since this transformation, there are still hints of what once was. Back in 2017, by comparing surveys of plant diversity with maps of archaeological sites at ancient settlements, scientists discovered that there are still higher densities of domesticated trees -- such as Brazil nut trees -- near where people lived all those centuries ago.

This suggests that gardens can still have a legacy, long after they cease to be recognisable. And though they may benefit from our attentions, garden plants are often perfectly happy when they're left alone.

In 2020, the head gardener at Wentworth Woodhouse in Yorkshire became intrigued by the mysterious purpose of a long-derelict corner of the estate -- a sandstone building with grand Palladian features and large windows, built in the 19th Century. At the time, the building was too dangerous to enter, because its glass roof was hanging off in lethal shards.

After consulting experts and the public, Scott Jamieson made an astonishing discovery: it had originally been used to house camellias, and it still contained 19 healthy plants. The oldest had lived there since 1792. Among them, he found several which are extremely rare -- collectively, the discovery was compared to stumbling across a library of first-edition books.

In fact, though we tend to think of trees as the most long-lived plants, they're not even close to holding the record. That honour is currently thought to be held by King's Holly, a shrub with shiny, spiky foliage and deep pink flowers that's native to a small patch of land in Tanzania. It reproduces asexually, and one critically endangered colony is an estimated 43,000 years old.

And if all else fails, some gardens live on in legends, if not in reality.

The Hanging Gardens of Babylon are possibly the most famous in world history -- revered for millennia as one of the Seven Wonders of the Ancient World. The problem is, though the city of Babylon was first excavated as early as 1811, and has been intensively studied ever since, no one has ever found any trace of them -- not one stump or brick. In fact, they're the only ancient wonder that has been disputed by historians. The mystery has been described as "baffling".

But as it happens, there is another way. In 1994, the historian Stephanie Dalley suggested that the gardens at Nineveh, and those that supposedly existed at Babylon, are the same. The two cities, 300 miles (482km) apart, were routinely mixed up in ancient sources because both were known by the same nickname, "the Gate of the Gods," as were several others. Later on, further blunders arose from nonsensical translations of Assyrian scripts in the 1920s.

Other evidence comes from jarring facts, such as the ancient Greek historian Herodotus's supposed visit to Babylon in the 5th Century BC, after which he didn't mention the hanging gardens even once. As Dalley points out, it would be surprising if he -- or whoever's information he relied upon to write about the city -- forgot to mention its main attraction.

By 2013, Dalley had amassed a portfolio of further evidence and wrote up her theory in a book, The Mystery of the Hanging Garden at Babylon. It's now widely considered to be a plausible explanation for the total absence of any archaeological evidence.

So, though the gardens at Nineveh survived just a handful of decades, they may have achieved a kind of immortality after all -- just under a different name.