Detectives are said to be confident ex Cistercian monk Joe Lynskey's remains are near Coghalstown, Co Meath, in the Irish Republic.
He went missing from west Belfast in August 1972 and is one of the so-called 16 'disappeared'.
The search is being carried out by the Independent Commission for the Location of Victims' Remains, which was set up to trace people killed and secretly buried by republicans during the Troubles.
The IRA has already admitted it abducted, killed and buried him.
Lead forensic scientist Geoff Knupfer said: "We have good reason to believe that we are in the right area and that his remains lie somewhere in the six hectares site but... we still have a difficult and complex task ahead of us,
"We are satisfied that the time is right to bring in our team of forensic archaeologists to start the search beneath the surface."
The scientist also urged people with any information to come forward: "There are people who have information who have not come to the ICLVR even though there is a guarantee of complete confidentiality,"
"It would be quite wrong to think that because we have started the search for Joe Lynskey that there is no need for someone who knows something of the events in Coghalstown in 1972 to give us that information.
"It might be the key to finding his remains and returning them to his family for a Christian burial."