Deep in the Cambodian jungle lie the remains of a vast medieval city, which was hidden for centuries. New archaeological techniques are now revealing its secrets - including an elaborate network of temples and boulevards, and sophisticated engineering.

In April 1858 a young French explorer, Henri Mouhot, sailed from London to south-east Asia. For the next three years he travelled widely, discovering exotic jungle insects that still bear his name.

Today he would be all but forgotten were it not for his journal, published in 1863, two years after he died of fever in Laos, aged just 35.

Mouhot's account captured the public imagination, but not because of the beetles and spiders he found.

Readers were gripped by his vivid descriptions of vast temples consumed by the jungle: Mouhot introduced the world to the lost medieval city of Angkor in Cambodia and its romantic, awe-inspiring splendour.

"One of these temples, a rival to that of Solomon, and erected by some ancient Michelangelo, might take an honourable place beside our most beautiful buildings. It is grander than anything left to us by Greece or Rome," he wrote.

His descriptions firmly established in popular culture the beguiling fantasy of swashbuckling explorers finding forgotten temples.

Today Cambodia is famous for these buildings. The largest, Angkor Wat, constructed around 1150, remains the biggest religious complex on Earth, covering an area four times larger than Vatican City.

It attracts two million tourists a year and takes pride of place on Cambodia's flag.

Find out more Follow the archaeological team in Cambodia as they uncover the mysteries of Angkor Wat. Watch Jungle Atlantis on Thursday 25 September at 20:00 BST on BBC Two or catch it later on the BBC iPlayer. Jungle Atlantis

But back in the 1860s Angkor Wat was virtually unheard of beyond local monks and villagers. The notion that this great temple was once surrounded by a city of nearly a million people was entirely unknown.

It took over a century of gruelling archaeological fieldwork to fill in the map. The lost city of Angkor slowly began to reappear, street by street. But even then significant blanks remained.

Then, last year, archaeologists announced a series of new discoveries - about Angkor, and an even older city hidden deep in the jungle beyond.

An international team, led by the University of Sydney's Dr Damian Evans, had mapped 370 sq km around Angkor in unprecedented detail - no mean feat given the density of the jungle and the prevalence of landmines from Cambodia's civil war. Yet the entire survey took less than two weeks.

Their secret?

Lidar - a sophisticated remote sensing technology that is revolutionising archaeology, especially in the tropics.

Mounted on a helicopter criss-crossing the countryside, the team's lidar device fired a million laser beams every four seconds through the jungle canopy, recording minute variations in ground surface topography.

The findings were staggering.

Image copyright Khmer Archaeology Lidar Consortium Image caption Lidar technology has revealed the original city of Angkor - red lines indicate modern features including roads and canals

The archaeologists found undocumented cityscapes etched on to the forest floor, with temples, highways and elaborate waterways spreading across the landscape.

"You have this kind of sudden eureka moment where you bring the data up on screen the first time and there it is - this ancient city very clearly in front of you," says Dr Evans.

These new discoveries have profoundly transformed our understanding of Angkor, the greatest medieval city on Earth.

Image copyright Getty Images Image caption Phra Sav Ling Povn, palace of the leprous king, near Angkor Wat, circa 1930

At its peak, in the late 12th Century, Angkor was a bustling metropolis covering 1,000 sq km. (It would be another 700 years before London reached a similar size.)

Angkor was once the capital of the mighty Khmer empire which, ruled by warrior kings, dominated the region for centuries - covering all of present-day Cambodia and much of Vietnam, Laos, Thailand and Myanmar. But its origins and birthplace have long been shrouded in mystery.

A few meagre inscriptions suggested the empire was founded in the early 9th Century by a great king, Jayavarman II, and that his original capital, Mahendraparvata, was somewhere in the Kulen hills, a forested plateau north-east of the site on which Angkor would later be built.

But no-one knew for sure - until the lidar team arrived.

The lidar survey of the hills revealed ghostly outlines on the forest floor of unknown temples and an elaborate and utterly unexpected grid of ceremonial boulevards, dykes and man-made ponds - a lost city, found.

Image copyright Khmer Archaeology LiDAR Consortium

Most striking of all was evidence of large-scale hydraulic engineering, the defining signature of the Khmer empire.

By the time the royal capital moved south to Angkor around the end of the 9th Century, Khmer engineers were storing and distributing vast quantities of precious seasonal monsoon water using a complex network of huge canals and reservoirs.

Harnessing the monsoon provided food security - and made the ruling elite fantastically rich. For the next three centuries they channelled their wealth into the greatest concentration of temples on Earth.

One temple, Preah Khan, constructed in 1191, contained 60t of gold. Its value today would be about £2bn ($3.3bn).

But despite the city's immense wealth, trouble was brewing.

At the same time that Angkor's temple-building programme peaked, its vital hydraulic network was falling into disrepair - at the worst possible moment.

The end of the medieval period saw dramatic shifts in climate across south-east Asia.

Tree ring samples record sudden fluctuations between extreme dry and wet conditions - and the lidar map reveals catastrophic flood damage to the city's vital water network.

With this lifeline in tatters, Angkor entered a spiral of decline from which it never recovered.

In the 15th Century, the Khmer kings abandoned their city and moved to the coast. They built a new city, Phnom Penh, the present-day capital of Cambodia.

Life in Angkor slowly ebbed away.

When Mouhot arrived he found only the great stone temples, many of them in a perilous state of disrepair.

Nearly everything else - from common houses to royal palaces, all of which were constructed of wood - had rotted away.

The vast metropolis that once surrounded the temples had been all but devoured by the jungle.

Watch the first episode of Jungle Atlantis on Thursday 25 September at 20:00 BST on BBC Two, or catch it later on the BBC iPlayer. The programme was made in association with The Smithsonian Channel, which will be transmitting both episodes in the US on 5 October under the title Angkor Revealed.