What on Earth are those? I thought to myself shortly before landing in the Iranian city of Esfahan one summer. From the aeroplane window, I could see what looked like a cross between freakishly large anthills and obscure symbols left by an extraterrestrial race. Little could I - then only a teenager - have guessed what lay beneath their mysterious surfaces.
The ancient Iranians had a huge task not only to survive, but also to conquer almost all their then-known world
If, throughout the ages, there's one element my people have revered more than fire - known as the 'Son of God' in the ancient Iranian faith of Zoroastrianism - it's undoubtedly water. The Iran in which various Aryan tribes settled millennia ago was a rich, vast and variegated expanse of land, as it is today. It also, however, happened to be incredibly dry. The ancient Iranians had a huge task not only to survive, but also to conquer almost all their then-known world.
To find pure water in an arid and unforgiving landscape, and create lush vistas in (literally) the middle of nowhere, might have seemed an impossible undertaking. However, they found an effective and sustainable solution to Iran's dearth of easily accessible water in the marvel of ancient Iranian engineering known as the kariz, more popularly known by its Arabic name, the qanat. Dating back some 3,000-odd years, and added to Unesco's World Heritage list in 2016, the qanat is a testament to the ingenuity of the ancient Iranians.
Simply put, a qanat is an underground channel that carries fresh water from an elevated source in the mountains to an opening at a lower altitude for the purposes of irrigation - a perfect solution in a region with an abundance of mountains. Once a possible source of fresh water is identified, indicated by the presence of an alluvial fan (a triangle-shaped sedimentary deposit at the base of a mountain), a shaft like one of the 'anthills' I'd seen is bored underground until the water source is reached. While in some cases not much digging is required, other shafts can extend up to 300m below ground. Numerous other anthill-like shafts are then bored at regular intervals to extract soil as well as provide ventilation for the workers excavating the earth below. To make things even more difficult, the slope of the qanat must be precisely calculated: too steep an incline, and the water's downward force will erode the qanat; too flat, and the water won't flow.
The complex system was well worth the effort, however. These underground aqueducts have allowed Iranians for millennia to access and transport water in some of its most arid regions. One of the most impressive examples is in the Fars province of south-west Iran. Here, the city of Persepolis was built by the Achaemenid Persians (550-330BC) in a hot and dusty plain surrounded by the Zagros Mountains. The location was not exactly endowed with nature's bounty. Yet, by way of the qanat, Persepolis became the epicentre of an empire that stretched from Greece to India, and was regarded by many as the most luxurious city in the world, famed for its opulent palaces and exquisite gardens. As such, it's easy to see why the distinctly Iranian hue of blue - known as abi in Persian, literally meaning 'water-like' - is nothing short of ubiquitous throughout the country.
The qanat system was so effective that it soon spread to other corners of the world, first through the ancient Persians' conquests, and later by way of the Muslim Arabs, who adopted the system from the Persians and carried it with them as far as Andalusia, Sicily and North Africa. According to William B Hemsley in The Qanat: An Ancient Water Supply, so highly did the ancient Egyptians value the qanat system that the Persian emperor Darius the Great "was later honoured with the title of Pharaoh" in return for introducing it to them.
Play it cool
Not only does the qanat provide necessary drinking water, it also helps lower indoor temperatures. In Yazd in central Iran, where summers can be stiflingly hot, the qanat is as indispensable as it is ingenious. Used in combination with a badgir (an Iranian wind-catcher), the water in the qanat cools warm incoming air, which enters it through a shaft, before being released into a basement and expelled through the openings at the top of the badgir. In houses in Yazd, for instance, this ancient method of air conditioning is still widely used, and is an inseparable aspect of engineering and architecture.
Similarly, the qanat made it possible to store large quantities of ice year round in desert climates. Constructed in conical shapes made of an admixture of heat-resistant materials, and also making use of Iranian wind-catching technology, the yakhchal (literally 'ice pit') is an ancient Iranian form of refrigeration dating to around 400BC. In the winter months, water would be sourced from a qanat and left to freeze in the yakhchal's basement enclosure before being cut into blocks and stored for year-round use. Air entering through the qanat shafts and cooled by the underground water would further assist in reducing temperatures.
'We must cultivate our garden'
But the qanats weren't only for physical sustenance; they also served a spiritual purpose. Despite their harsh environs, through this feat in engineering the ancient Persians were able to construct the renowned, Unesco-listed Persian garden.
Heavenly to behold and enjoy - in stark contrast to the parched surroundings - these lush oases, often arranged in four sections as a chahar bagh (literally 'four gardens') - are replete with trees, flowers, fountains and waterways, all meticulously arranged in harmony and symmetry to reflect the Zoroastrian adoration of nature and the elements. It's not surprising that descriptions of paradise in the Abrahamic faiths have their origins in the Persian garden, which the Persians called pari-daida (denoting a walled garden), from which the word 'paradise' derives.
The qanat is as indispensable as it is ingenious
According to Iranian Studies scholar Touraj Daryaee, the ancient Persian gardens "held every sort of plant and flower, irrigated by running water, a most precious commodity for the inhabitants of the plateau." The Bagh-e Shazdeh (Prince's Garden) near Kerman in central Iran is a dazzling example: seen from above, it beggars belief that such a wonderland of greenery and gushing springs could exist surrounded by nothing but parched earth and rugged mountains. But gardens - which are, by and large, open to the public - can be seen all around the country.
When in my native Tehran, I can often be caught sipping on traditional Iranian chai, savouring the picturesque scenery, oblivious to the hustle and bustle on the outside streets, at the Bagh-e Khoshnevisan (Calligraphers' Garden), Bagh-e Muzeh (Museum Garden) and Bagh-e Ferdows (Paradise Garden), all of which are in the north of the city. I mainly visit to escape the stifling summertime smog, and to enjoy the sound of fountains and singing birds amid leafy plane trees, shrubbery and flowerbeds, all of which evoke the florid poetry of Hafez and Sa'di.
As with the qanat, the Persian garden not only continues to thrive in modern-day Iran - where it also informs much of carpet-making in terms of layout, design and themes - but also elsewhere around the world. The impact of the chahar bagh philosophy can be seen as far away as Versailles in France, the gardens and courtyards of the Alhambra and the palaces of Marrakech, having been imported by the Arabs in the case of the latter two.
However, the best examples outside Iran perhaps belong to Mughal-era India and Pakistan. Just as the Mughals considered Persian the apex of refinement when it came to language, so too did they the Persian garden where horticulture and landscape architecture were concerned. Using the chahar bagh as a template, the gardens of the Taj Mahal and Humayun's Tomb, for example, were naturally called chahar baghs by the Mughals, and still are today.
A steady flow
Although technological innovations have reduced the reliance of Iranians on the qanat, the aqueducts are still prominent and widespread throughout the country. With tens of thousands of qanats in Iran today boasting a total distance comparable to that between the Earth and the moon, the ingenuity of the ancient Persians has more than stood the test of time. In accordance with the ancient Iranian/Zoroastrian reverence for nature and the elements, it is incomparable as a sustainable and environmentally friendly method of not only fresh water extraction, but air conditioning and refrigeration. As well, in rural contexts, it allows for the equitable distribution of water, and through the necessity of its ongoing maintenance brings about social collaboration.
Despite Cyrus the Great's world renown as a wise, just and compassionate leader, one can't help but wonder what the empire he founded - the basis of modern-day Iran, barring politics - would have been without the aid of the qanat. What would the Persian army and people have done without access to fresh water? What would Persepolis - and the innumerable cities in the vast dominion of the Empire - have looked like, devoid of the pleasure gardens that forever changed the face of horticulture and landscape architecture?
The ingenuity of the ancient Persians has more than stood the test of time
For a people who, according to scholar of religion Bruce Lincoln, "... meant to conquer the entire known world in the name of establishing Paradise on Earth", as Daryaee reports him to have posited, doing so from Iran's often ruthless climate without the wonder of engineering that is the qanat might have been little more than a pipe dream.
If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital and Travel, delivered to your inbox every Friday.