South African historians are searching for a gun buried by Nelson Mandela in Johannesburg before his 1963 arrest.
Mr Mandela buried the gun 1.5m below ground
The former South African president has told how he buried the weapon on the farm where he was hiding at the time.
Other ANC leaders were later arrested on the farm - Mr Mandela was arrested elsewhere. All were jailed for their work against apartheid.
Nicholas Wolpe, head of the trust which is carrying out the search, said he was confident that the gun would be found.
"I am 90% convinced that gun has never been found," said Mr Wolpe, the son of former ANC leader Harold Wolpe and chief executive of the Liliesleaf Trust.
Mr Wolpe told the BBC News website it was only in April 2003 that Mr Mandela had told him about the hidden weapon during a visit to the site of the former Liliesleaf Farm in the suburb of Rivonia.
"I was walking outside the main house when Nelson turned to me and asked: 'Have you found the gun?'" Mr Wolpe recalled.
"We weren't aware there was a gun buried on the property."
The weapon was a Makarov pistol given to Mr Mandela by an Ethiopian army officer when he was undergoing military training in Addis Ababa.
Mr Mandela gave a clue as to the burial spot by saying it was "50 paces from the kitchen" - but this initially led to confusion as he was referring to a second kitchen in a part of the house that was subsequently demolished
Since 1963, the property has been subdivided and houses built on what was once farmland.
Funded by government and the private sector, the Liliesleaf Trust has been buying up properties with a view to restoring the historical site where the ANC leaders once hid, and creating an educational centre.
The place where the gun is thought to lie was under the foundations of a house that was demolished after the trust bought the property.
Mr Mandela would have been trained in how to conceal weapons, Mr Wolpe said, and buried the weapon 1.5m (five feet) below the surface - lower than the excavations for the foundations of the house subsequently built on the site.
Since this is beyond the range of a normal metal detector, the search team is currently conducting a systematic search with a mechanical digger.
The former president also said he had covered the weapon with an aluminium sheet.
"We know that as soon as we get aluminium, we've found the gun," Mr Wolpe said.