The latest search for Columba McVeigh is starting on the 21st anniversary of his father's death.
Paddy McVeigh died without ever knowing where his son's remains are buried.
His wife Vera passed a decade later, again without answers.
The grave where they lie also has Columba's name on it - the enduring hope being he will one day join them.
The IRA murder victim's sister said she hopes the new dig on Bragan bog in Co Monaghan will finally end the family's torment.
Dympna Kerr said her father's anniversary had added to the "emotional turmoil" of the fresh excavation.
Mrs Kerr, who lives in Merseyside, said: "For years I couldn't bring myself to stand on that bog where we were eventually told that Columba was buried.
"I went to it earlier this year and it's a bleak and desolate place and I cannot bear the thought that Columba is out there when he should be beside our mum and dad.
"Knowing that the ICLVR (Independent Commission for the Location of Victims' Remains) will do everything they possibly can to bring Columba home and also knowing that we have the thoughts and prayers of the other families of the Disappeared gives me the strength to face the coming days and weeks.
"It's 21 years ago today that dad died not knowing where Columba was buried.
"As every year passes we ask 'for how much longer must this go on?'
"I can only hope and pray that the torment will end."