Over the next three years, in time for Hanoi's 1,000th birthday in 2010, scientists intend to clean up Hoan Kiem Lake, home to the creature that symbolises Vietnam's centuries-old struggle for independence.

Vietnamese and German experts say they will use a new device, which borrows from the designs of corkscrews, submarines and tanks, to suck several metres (feet) of toxic sludge from the bottom of the 'Lake of the Returned Sword'.

The 2.4-million-dollar project will be a delicate one.

The famed, algae-green lake is home to an elusive turtle that is a key figure in Vietnam folklore.

In a story that every Vietnamese child learns at school, the 15th century farmer-turned-rebel leader Le Loi used a magical sword to drive out Chinese invaders and found the dynasty named after him.

When Le Loi, by now the emperor, went boating on the lake one day, a turtle appeared, took his sacred sword and dived to the bottom of the lake, keeping the weapon safe for the next time Vietnam may have to defend its freedom.

Today, occasional sightings of a giant soft-shell turtle draw large crowds, and photographs and amateur video clips attest to the claim that at least one turtle indeed still lives in the lake.

The turtle legend is a staple of traditional water-puppet theatre, and reported sightings of the animal, a symbol of eternity, are deemed auspicious, especially when they coincide with major national events.

"Since 1991 the turtle has come up about 400 times," said Vietnam's pre-eminent authority on the animal, Professor Ha Dinh Duc of the Hanoi University of Science -- better known here as the 'turtle professor.'

"Several times when it came up, it coincided with important events," he told AFP. "It's something we can't explain."

The turtle has appeared when Chinese presidents have visited, during the inauguration of a Le Loi statute, at the start of last year's Communist Party congress, and even during a conference on endangered reptiles, Duc said.

The professor says he doesn't know the age of the turtle -- which he says is a new species he has named Rafetus Leloiiis. He says it weighs around 200 kilogrammes (440 pounds).

-- "We have to take care of the turtle" --

Previously, at least four of the turtles lived here -- one of them is now stuffed and on display in an island temple on the lake -- but today only one is left and Duc frets about its well-being.

From his Hanoi home, crammed with turtle books, pictures and paraphernalia, he has pushed for efforts to save the turtle, also proposing to catch animals of the same species from another pond to mate with it.

"There has been coffee shop talk about cloning the turtle," he said, "but I would oppose it."

The more immediate threat to the turtle is man-made.

Stormwater run-off from the growing city has sullied the stagnant lake with chemicals and organic pollutants that feed algae blooms and choke off oxygen.

"The water quality is decreasing, and we expect a breakdown of the aquatic habitat within a decade," said Professor Peter Werner of Germany's Dresden University of Technology. "The lake could be dead in 10 years."

Hoan Kiem Lake, about 600 metres long and 200 metres wide, is now only about 1.5 metres deep while a four-to-six-metre deep layer of sludge has accumulated on the lake bed, said Christian Richter of German company HGN Hydrogeologie.

German scientists have developed an "subaquatic vacuum cleaner" that will crawl along the lake floor using two corkscrew-like spirals that dig up and funnel the mud into a pipe while also propelling the device forward.

The remote-controlled "SediTurtle" will use buoyancy to rise and sink like a submarine and use brakes on its two coils to move left and right like a tank, said engineer Dr Frank Panning of company GSan oekologische Gewaessersanierung.

"We are using low-impact environmental technology that is silent and minimises turbulence and the release of toxic compounds," said Werner. "This project is very sensitive. We have to take care of the turtle."

In the first phase, set to start early next year and take 24 months, scientists will first analyse water and sediment samples from Hoan Kiem and test the SediTurtle in another Hanoi lake.

If all goes well, Vietnamese experts could then take over and use the new technology to clean up the famous lake itself, said Werner.

Professor Duc -- whose support is deemed crucial for any project involving Hoan Kiem Lake -- has given the green light after vetoing earlier offers for help from Japan, Thailand and elsewhere.

"Many international organisations have offered to help," he said.

"This project is environmentally sound, and it's good for the turtle. And the turtle is important for Vietnam."

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