At first glance, it is business as usual at the great sandstone temple of Angkor Wat. Through a drape of evening haze, the ancient Cambodian superstructure sees another batch of tourists process across its moat and marvel at its grandeur. Local teenagers waggle cool drinks in the faces of passers-by and auto-rickshaw or "tuk-tuk" drivers loudly vie for business. It looks like what it is – a boom town.

But the modern commercial success of the high-profile complex, on the site of the ancient city of Angkor, may be – literally – on shaky ground.

According to heritage experts carrying out restoration work at the temple, which is one of the biggest sets of religious ruins in the world, a plethora of new hotels, cashing in on the country's near-exponential rise in tourist numbers, is sapping gallons of water from beneath nearby urban areas. They say this could upset the delicate foundations on which Angkor Wat sits and could lead to parts of it – including its famous celestial apsara, or carved nymphs – taking an unheavenly tumble to earth.

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Philippe Delanghe, the culture programme specialist at Unesco's Phnom Penh office, said this week: "There is a very important balance between the sand and water on which the temple is built. And if that balance is taken away then we might have trouble with collapse.

"The growth in the number of hotels around Angkor Wat has meant that more and more holes are being drilled into the earth to extract water from the water table. And this has profound consequences for this important mix.

"We saw something similar with the weakening of the stability of ruins in Indonesia two years ago, and there is the possibility that we will see something like this here." Mr Delanghe added that the long-term consequences of unstable ground beneath the monument could include cracked ceilings and falling pillars. "If it becomes so damaged then we will have no tourists," he added.

Locally, it is easy to see why comments such as these go down as badly as, say, a tumbling nymph. The temple, which is venerated enough to appear on the national flag, is the jewel in Cambodia's heritage crown. Not only is it in the best condition of any such structure at the Angkor site: it has been tightly linked with Cambodia's history for nearly a millennium. It is thought to have been built as a funerary temple for King Suryavarman II (who died in 1152) to honour Vishnu, the Hindu deity with whom he identified. The sandstone blocks from which it was constructed were quarried more than 30 miles away and floated down the Siem Reap river. Recent research suggests that Angkor – of which this temple was surely the centrepiece – was an urban settlement that covered some 700 square miles, comparable in size to Greater London, and therefore the world's largest medieval city.

With cultural attractions like this, it is little wonder that tourism is such an important source of revenue for the impoverished and (until 10 years ago) war-blighted nation. In many parts of Cambodia, a £100 annual income is still enough to live on.

In 1993, when Angkor was first added to Unesco's World Heritage List, the militant Khmer Rouge were still active in certain areas. Just 7,600 souls ventured to the temple complex that year. Since then, however, Cambodia has become "safe" in the eyes of the international community, and package tours have landed in fleets. In 2007, about two million tourists visited Cambodia, with half stopping at Angkor Wat. With tourist traffic continuing to increase by about 20 per cent year on year, some three million people are expected to visit the country in 2010. The temple has become a must-see stop on any tour of south-east Asia.

Such popularity is, clearly, a mixed blessing. At the nearby town of Siem Reap, there are now hundreds of garish hotels and guest-houses, with many more in the pipeline. Wealthy travellers can stay in the centrally located Le Meridien, replete with its Romanesque swimming pool, for about $300 (£150) a night, before heading out for a swift nine holes at the Nick Faldo-designed Angkor Golf Resort. Most of the restaurants serve Western food, and the Associated Press has reported that several notable old buildings have been razed to the ground to make room for fresh accommodation. "The identity Siem Reap had for centuries is gradually disappearing, or maybe almost disappeared," Teruo Jinnai, the director of Unesco's office in Cambodia, said recently. "You have restaurants, massage parlours, hotels, and it's very sad to see that." The crowds also take their toll on the local built heritage. "Pollution from buses, which often leave their engines running to supply air conditioning, will cause darkening of the stone," said Mr Delanghe. Further to this, the five-tiered Phnom Bakheng hilltop temple, an important relic of the ancient Khmer civilisation like Angkor Wat, sees some 3,000 tourists climb up its narrow stone staircases every evening. At its summit people can view a picturesque sunset over Angkor Wat. But on the way there, the punters are wearing through its precious sandstone carvings, which they use as handholds during the climb. John Stubbs, of the World Monuments Fund, said last year: "It simply cannot survive this daily assault. Unless it is completely closed off for essential repairs, Phnom Bakheng will suffer critical damage."

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The growth of Siem Reap is causing other problems, too. "This tremendous growth added to population increase has been exacerbating pressure on infrastructure," a 2005 World Bank report said. "Energy, water, sewage and waste are all significant problems... Most guesthouses reportedly dump used water directly into the river, causing noticeable river pollution." E. coli, the bacteria found in human faeces, has begun seeping into local wells.

Sadly, the most high-profile victim of Cambodia's success in tourism could well be Angkor Wat itself. The World Bank report added that "one of Angkor's temples is reportedly falling into a sinkhole, suggesting that the underground aquifers may be rapidly disappearing". The monument in question was the Bayon temple, famous for the delicate faces carved on to its 54 towers. It is reportedly still collapsing into the sandy ground and visitors can observe its sinking foundations and widening cracks. The cultural specialists now claim that if it is happening there, it could happen anywhere.

Mr Jinnai said this week: "Test wells have conclusively proven that the ground water level is definitely lowering in Siem Reap town. While it is not yet considered at a dangerous level, if the water intake doubles or triples then it is going to get really dangerous around the city and this could well affect historic monuments such as Angkor Wat in the long term."

And even the former Japanese ambassador Fumiaki Takahashi, who during his recently ended tenure, bemoaned the lack of control surrounding water extraction in Siem Reap. It was the Cambodian government's "urgent task" to control such things, he explained, because "if you take too much water, it might affect the Angkor site. In the long run, the underground water will go down and the site would sink."

Soeung Kong, deputy director general of the Aspara Authority, which oversees Angkor's upkeep, has urged people to keep a level head about this potential damage. He told Agence France Press: "The harm to the temples is unavoidable. We are trying to keep that harm to a minimal level."

The Japan International Cooperation Agency (Jica) is collaborating with Unesco over recommendations for water management. One Jica report has suggested the removal of underground water from near Phnom Kraom, a hill near the edge of the Tonle Sap lake about 7.4 miles to the south, as an alternative to the town's own groundwater. The plan has won the backing of Cambodia's deputy tourism minister, Thong Khon.

Another solution put forward is the promotion of temples away from Angkor Wat, to relieve the pressure on it. This will be helped as infrastructure networks are built, such as a new road to Koh Ker, which boasts a collection of temples and towers to the north of Siem Reap. Another idea being considered by officials would be to limit access to the temple compound using a reservation system; such solutions would at least minimise the risks of direct damage by tourists.

Any of these systems could combine well with Heritage Watch, which promotes sustainable tourism in Siem Reap. The organisation certifies local companies as "heritage-friendly" or not. Such a label indicates the adherence to such things as clean environmental policies, support for the local economy, or contributions toward preservation. But overall, Mr Delanghe stressed that much more research was necessary before any particular path could be backed.

Unesco is due to come up with its final suggestions for the future in May. In all likelihood these recommendations, Mr Delanghe said, would include the suggested assembly of a central database to monitor the amount of water being extracted in Siem Reap. "As long as we don't have the data we don't know what is going on," he warned.

But back at Angkor Wat, we do. Dusk has fully descended. Tourists are being expelled from the site, and are being told, helpfully enough, to stop sitting on the ruins, lest they damage them with their rucksacks. One by one they file out from its inner sanctum: past the Statue of Vishnu, through the suitably gargantuan Elephant Gate, to make their careful way back across the sandstone causeway to the parked cars beyond. For now, the temple's future remains secure. The fleet of vehicles start their engines, fire up their air conditioning, and slowly make their way back to the growing sea of neon that is Siem Reap.