Experts showed off the German-developed sediment-eating machines this week as part of preparations before an expected cleanup of Hoan Kiem Lake, the heart and soul of Vietnam's capital.

Nestled in the centre of rapidly urbanising Hanoi, the so-called Lake of the Returned Sword is home to an elusive turtle which symbolises Vietnam's centuries-old struggle for independence.

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.

Reported sightings of the turtle, a symbol of eternity, are deemed auspicious, especially when they coincide with major national events.

The site's historical importance therefore requires a delicate clean-up operation, and the joint team of Vietnamese and German experts have been planning how to clean the turtle's home with minimum risk to the creature.

Leonhard Fechter, of Berlin's Herbst Umwelttechnik GmbH, said he knows people care about the turtle, so his company made the SediTurtle with a 'soft' technology that will not harm the animal.

A second, more sophisticated SediTurtle resembles a giant corkscrew and operates by remote control 'like a submarine", said Peter Werner, a professor from the same Dresden university.

This less obtrusive option is Mr Werner's preferred choice because it would not need unsightly ropes to pull it around the picturesque Hoan Kiem lake.

Hoan Kiem has suffered the fate of all lakes. Over time, sediment increases and the water level drops, particularly in urban areas. -- AFP