Police say they believe the body of missing man Jeff Mundy is at the bottom of the Myponga Reservoir, about 60 kilometres south of Adelaide.
SA Police are currently conducting a search in what Detective Superintendent Des Bray said were "very challenging" conditions at a depth of more than 40 metres.
Superintendent Bray said they had information that Mr Mundy's body was dumped near the weir, but had also received reports that his remains may be at another undisclosed location.
"The search will be very challenging for the divers — they'll be operating in 42 metres of water, it'll be cold, very limited visibility," he said.
"There's a significant silt build-up at the bottom and they'll be feeling through the bottom of the dam … for his remains in what amounts to complete darkness.
"If Jeff is there or evidence associated with his death, we will find him.
"If he's not there, that will still aid the investigation in that we will be able to eliminate this particular site."
Mr Mundy was last seen getting off a bus at Sellicks Beach on December 19, 2020, and police declared his death a major crime last year.
He was aged 36 at the time of his disappearance, which is being investigated as part of Taskforce Southern, a police operation launched in 2021, to investigate suspected drug-related murders in Adelaide's south.
Divers undergoing decompression
Police believe Mr Mundy, Trevor King and Robert Atkins were killed by a group of people over small drug debts.
Mr King was found dead in waters at West Lakes in January 2020, but the body of Mr Atkins has also not been found.
"We are still trying to work out a more defined area where we can find him," Superintendent Bray said.
"We have a reasonable indication as to where we think he might, be but not such a small area that we can go straight to it. We're hoping the situation with Robert might change in the coming weeks."
Divers are expected to remain at the Myponga Reservoir for five days, but SA Police said even if Mr Mundy's remains were not found there, detectives believed the "disposal site for his body is associated with a body of water".
Superintendent Bray said the water was so deep at the base of the dam wall, that the seven divers involved — who were taking it in turns to operate in pairs — needed to undergo decompression before resurfacing.
Taskforce Southern has so far led to 65 arrests, 72 property searches and the recovery of 40 firearms.
Police have previously revealed Mr Mundy was held against his will at a storage yard on Morrow Road at Lonsdale in the weeks leading up to his death, and on Monday gave further details.
"We know that Jeff was taken to Lonsdale, to Morrow Road, and held captive in a container for a period of time, and escaped from there and then was taken again later," Superintendent Bray said.
"We'll do everything humanly possible to find Jeff and Robert."
