A reservoir in West Dublin is being searched as part of the investigation into the disappearance of Thomas Kennedy.
The 81-year-old from Finglas was last seen three weeks ago putting his bins outside his home in the Virginia Park area.
Gardaí and his family launched a massive search operation but have so far been unable to trace the pensioner.
The scouring of a reservoir near Dunsink lane follows a trawl of the Royal Canal and Tolka river.