A BIZARRE underground clue could lead cops to finding out the fate of "Australia's Madeleine McCann" as they dig up his foster grandmother's garden.

       Three-year-old William Tyrrell vanished in 2014 with no trace of him ever found and no person ever charged.

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       Forensics have been at the house overnight on Tuesday sifting through the soil Credit: PA 6

       The presence of blowflies may indicate that there was decomposing body there Credit: PA 6

       William Tyrrell went missing seven years ago Credit: NSW Police

       Forensics specialists in Kendall, New South Wales, are searching for any indication of human remains in the garden of the boy's foster grandparents.

       Meanwhile, William's foster parents were charged in connection with the alleged assault of another child.

       There is no suggestion that they are guilty of William's disappearance, but his foster mother is now a person of interest, NSW Police said.

       Her car, which belonged to William's foster grandmother at the time of his disappearance, has been seized for examination by forensics who are also searching a second-storey balcony following a theory the boy may have fallen to his death.

       William was in the care of his foster parents who took him on a visit to the coastal town of Kendall on the day he disappeared.

       His parents went inside to make a cup of tea while William and his sister played hide and seek outside.

       When they returned five minutes later the toddler had vanished.

       A police investigation into William's disappearance at the time flagged two suspicious cars seen parked on the street earlier that day.

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       At the time, William's biological parents remained unaware for hours as their son was reported missing and a dramatic search for the three-year-old began.

       Despite seven years having passed since the little boy was last seen, detailed information can still be collected from the ground, it's been revealed, particularly if it contains evidence of animal larvae, according to forensic scientist from Murdoch University Paola Magni.

       While insect activity would have been at its highest in the early stages of human remains being detected in the environment, evidence of such activity could be found long after, Dr Magni said.

       “The cocoons of flies can stay in the environment for a very long time. There are cocoons, or pupa cases, that you can find in the environment of Egyptians, so centuries after [a body is buried],” she told news.com.au.

       BLOWFLIES SIGNAL ORGANIC MATTER IN SOIL

       “They can still provide information,” Dr Magni said, adding detectives may need to bring in a forensic entomologist to investigate the prevalence of pupa cases in the area.

       Insect pupae would be incredibly valuable if found because they can indicate historic presence of organic matter in the soil, even if there are no remaining visible traces, Dr Magni said.

       “With carrier insects you have blowflies and maggots, but when the maggots become blowflies, there is a metamorphic period they spend in a pupa case, and that case can be in the soil,” she said.

       “Normally they should not be there, so if you find that in a search, it could be something interesting to consider.”

       Blowflies would typically only be in the area if there was a source of food such as a decomposing body, Dr Magni added.

       Insect analysis was one of several techniques available to detectives working to find William, who vanished from the property in 2014 when he was three.

       Photos released this week showed officers using giant sieves to filter through soil, which forensic expert Professor David Ranson said was typical when searching for remains.

       Other methods including aerial photography and heat detection devices, as well as analysing vegetation regrowth and changes, ground contour changes, and changes to soil colours, were likely being used to find a potential site of interest.

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       The foster parents have been charged - but not in connection to William Credit: PA 6

       He vanished while in the car of his foster parents Credit: NSW Police 6

       They believe they are close to finding out what happened to him Credit: PA