Diver Dave Woodward and sonar operator Gregg Mikolasek had been boating on Lake Hovsgol (pronounced hervs-gerl) for hours when they saw it: a giant 'trace', or shape, suspended about 35 metres below the surface in the cool waters of the 4-million-year-old freshwater lake in northern Mongolia, on the Russian border. 'I think we saw something and I think it was about six metres long,' Woodward recalls telling his diving buddies back at the camp later that afternoon. About three metres wide in the middle, whatever was down there exhibited the half-moon shape typical of the image of a fish on sonar. Its thick central section glowed red, indicating density, another pointer that whatever it was, it was probably a single entity and not a school of fish. Woodward thought about going in, hesitated, then decided not to. 'Oooh, no,' says the Englishman, who works for a Beijing-based mining company and travels regularly to Mongolia on business. 'It was just too big. And the question is, what does that thing eat in order to get so big?' Rumours have long circulated about giant 'monsters' in the waters of Lake Kanas, 1,000km west and a little south of Hovsgol, at the very northern tip of China's Xinjiang province, also on the Russian border. Tourists post grainy pictures that may or may not show creatures below the surface of the 90-metre-deep waters of the lake; the latest, taken this summer from the steep mountainsides that surround the lake, purported to show enormous, swirling wakes that could conceivably have been caused by underwater creatures. But Woodward and Mikolasek's finding of a giant, unidentified suspended object - possibly a fish or other kind of creature, or even a cluster of very large fish - is the first ever such report from Lake Hovsgol, and it lends support to the idea the isolated, deep and ancient lakes of the region that encompasses northern and western Mongolia, Xinjiang and southern Russia may all harbour giant creatures about which we know almost nothing. For his caution, Woodward was 'sent to the trip tower', jokes Steven Schwankert, leader of the dozen-strong diving team that explored the 260-metre-deep waters of Lake Hovsgol in mid-August, the first scientific diving expedition ever to the 136km-long, 40km-wide body of water, which lies high up in the Baikal Rift zone. 'We were all furious with him,' says Schwankert. 'It was possibly the discovery of the year in cryptozoology [the study of creatures whose existence is not proven, such as the Loch Ness monster] and he didn't go in!' Others may find Woodward's caution understandable. Mongolians living on the shores of Lake Hovsgol, many of them reindeer herders, tell of 'underwater deer' that live in the lake. Shown the sonar image, the Lake Hovsgol park ranger, a Mongolian, immediately identified it as one of them, says Woodward. Around Lake Kanas, locals say the creatures sometimes emerge from the lake and seize small animals that wander too close to the shore. Certainly, says Schwankert, the sonar operator found the image 'very odd'. And Mikolasek is no amateur, being the man who found the crashed remains of a US B-29 Superfortress bomber in Lake Mead in Nevada in 2002. The plane, which crashed in 1948, lay in rough underwater terrain of submerged cliffs and canyons at a depth of 80 metres. The discovery was a major coup for Mikolasek and his company, In Depth Consulting. Describing the Hovsgol entity as having a 'rather an unusual sonar signature', Mikolasek adds: 'The initial reaction was very much, 'We need a bigger boat!'' Both men feared for the safety of their 15-foot craft. Yuan Guoying is a scientist at the Xinjiang Environmental Sciences Research Institute in the provincial capital, Urumqi. He has studied Lake Kanas for decades and made dozens of trips to the lake. He believes it is eminently possible there are giant fish in all the ancient lakes of the region. 'There should be this kind of thing over there [in Mongolia] too,' says Yuan. 'They might have the same creatures as we have in Lake Kanas.' What those are, exactly, is still up for debate. Yuan is the first to agree, in reality, the 'monsters' of Lake Kanas are probably enormous taimen, a member of the salmonid family that includes salmon and trout, which have flourished in the isolation of the ancient lakes. However, that doesn't fully square with some of his own sightings, which he insists are reliable, of creatures up to an astonishing 120 metres long. Yuan says that in 2004, on two separate occasions, he saw four-metre-long fish in the lake. Experts agree taimen could grow to that size. He also says that in 1985, standing on the steep hillsides around Lake Kanas, he looked down and saw a shape just below the water that was 10 to 15 metres long, which he describes as 'greenish'. Even more surprisingly, in May 1994, standing on the hillside again, Yuan says he saw eight black shapes, each up to 120 metres long. The sighting was made from a distance of about 1.5km. 'They were near the edge of the lake and I saw them really clearly - clearly enough to count them. They were absolutely huge. I literally couldn't believe my eyes. I thought they were soil run-off from the mountains, but then I watched them for a minute or two and they all disappeared. There might be more of them in there.' Professor Clyde Goulden is director of the Institute for Mongolian Biodiversity and Ecological Studies at the Academy of Natural Sciences in Philadelphia, in the United States. He has studied Lake Hovsgol's ecosystem for years, focusing on the impacts of nomadic herders and climate change in the region. This summer, Goulden was awarded Mongolia's Medal of Friendship by its president, Nambaryn Enkhbayar, in recognition of his scholarship and efforts in the region, which contains 70 per cent of Mongolia's fresh-water resources. Lake Hovsgol, says Goulden, is a precious resource not just for Mongolia, but for the planet. In an increasingly polluted world, its waters are squeaky clean. Herdsmen and travellers have long drunk straight from the lake, and tests carried out by Schwankert's expedition confirmed its pristine status. But it, too, is beginning to come under pressure, with large herds of deer and other animals overgrazing the area. Culling, commonly practised during the years when Mongolia was a satellite state of the Soviet Union, a period of about 70 years until 1990, was abandoned when the country achieved independence that year. Now, the livestock population around Lake Hovsgol numbers about 35 million, up from 20 million less than two decades ago, something Goulden describes as 'a huge, huge population'. Add to the overgrazing the effects of global warming, which is blamed for raising temperatures around Lake Hovsgol by a significant 2 degrees Celsius in the past decade, and the region is beginning to suffer from growing desertification. Both overgrazing and warmer temperatures encourage water evaporation from the soil, which then washes into the lake, muddying its waters. Ever larger livestock herds may raise incomes in the short term but Goulden believes it's the wrong economic approach. 'One of my goals is to help keep the lake as pristine as possible. It will do more good for Mongolia as a tourism destination than the smaller gains of grazing livestock in an area inaccessible to markets,' he says. Schwankert agrees Lake Hovsgol has enormous tourist potential. And in the future, questions about what lies in the lake's deep waters may only increase its appeal. A high-altitude lake, Hovsgol sits at 1,650 metres amid something akin to an alpine environment. The mountains offer beautiful walking. The area is not nearly as inaccessible as people imagine, says Schwankert; there are regular flights between Beijing and Mongolia's capital, Ulan Bator, which are less than two hours apart, and from there a local commuter plane takes visitors to the town of Moron. Lake Hovsgol lies four hours' drive away, over unpaved roads. 'It is so clean and so empty. It's hard to imagine today such a place can exist in the world, especially in crowded Asia,' he says. As well as looking for unidentified objects, Schwankert and his team dived down to Soviet-era boat wrecks. 'It was amazing, because you didn't know what you were going to see next,' he says. For Schwankert, one of three of the divers on the expedition who belong to the New York-based Explorers Club, it was a thrill. 'Even as an explorer you get these opportunities very, very rarely. It's something I will never forget. It was unreal, stuff that I dreamed about when I was three years old and used to watch [French underwater explorer] Jacques Cousteau on television.' Schwankert also runs Beijing's only diving club, Sinoscuba. The team had hoped to spot car wrecks in the lake, but failed to, probably because they lie too deep. Lake Hovsgol is frozen over for seven months of the year and in the deep of winter even large trucks take short cuts across the surface to save petrol. Sometimes, they fall through the ice. Locals estimate there are about 50 car and truck wrecks on the lake floor. Schwankert says aside from that, the expedition had three major goals: to test the purity of the water; look for Buddhist relics reportedly tossed into the lake during the decades of Soviet religious repression; and scout for those mysterious 'underwater deer'. They didn't find any Buddhist relics, but their reports of the giant underwater presence - the sonar imaged it twice - will fuel speculation. And there is another sign that points to the possibility of something large and alive being down there: the size of the ordinary fish spotted near the lake's surface. Diving at relatively shallow depths, of up to 30 metres, the team found unusually large fish - up to two metres in length. 'They weren't shy and would come right up to us,' says Schwankert. 'That's something you'll find in places where people don't eat them.' Woodward describes diving around a sunken Russian wooden boat and having a fish measuring more than one metre in length, 'as thick as this', he says, describing a round shape with his hands that is 25cm in diameter, shooting between his arm and body. 'It gives you a fright,' he says. Scientists so far know of nine species of fish in the lake, of which one, the Hovsgol grayling, is unique to its waters. But overall, little is known about the lake's aquatic inhabitants. That is unsurprising, says Goulden. Mongolians traditionally don't eat fish, despite the presence of a large number of both fresh and saltwater lakes. Partly, that's because of the country's Buddhist and animist past. During the Soviet era, both religions were savagely suppressed, but they are making a comeback. 'A lama there explained it to me once. He told me the fish are the owners of the lake and you don't eat the owners of a home,' says Goulden. As a scientist, Goulden is understandably careful to play down talk of monsters. Yet he agrees unexpected discoveries may lie ahead. Lake Hovsgol is unusual in that its known species of fish are all, bar one, very large, he says, compared to 'most lakes, [where] you find mostly small forms and perhaps one large one'. Another characteristic is the lake's beds of marl, a type of carbonate, which lie about 40 metres deep. 'You tend to have unique kinds of animals and plants in marl,' says Goulden. 'Lake Hovsgol is poorly studied and there is certainly no reason to think we've discovered everything in the lake. There's really ample opportunities for new discoveries.' Goulden points out that while taimen - the probable inhabitants of Lake Kanas - live in the Egiyn River, which flows out of Lake Hovsgol, as far as anyone knows they have never managed to enter it. (The Egiyn flows into the Selenge, which empties into Lake Baikal, the oldest and, at 1,600 metres, the deepest lake in the world. In contrast to the one river that empties Lake Hovsgol, about 100 rivers flow into it.) 'The taimen cannot enter and survive in Hovsgol,' says Goulden. 'They are more of a river fish. But there are other fish in the lake, like lenok [a kind of Siberian trout], that can be up to three-quarters of a metre long. It may be there are species close to the bottom that cannot be caught.' For Mikolasek, seeing is believing. 'Until someone actually sees this thing, it will remain a mystery.'