The wreckage of Coast Guard helicopter Rescue 116 has been located off the Mayo coast.
The main part of the aircraft was detected by underwater cameras this morning, around 60 metres off Blackrock Island.
It has been located at the site where a signal from the helicopter's black box recorder was detected last week.
The development has been described as "highly significant".
No bodies have been recovered at the scene but investigators have stressed that more detailed examination of the wreckage has yet to take place.
They are hopeful that the three missing crew members may still be in the helicopter.
Captain Mark Duffy, winch operator Paul Ormsby and winchman Ciarán Smith remain missing since the helicopter crashed last Tuesday.
Crew member Captain Dara Fitzpatrick was transferred to Mayo University Hospital after being recovered from the water in a critical condition by an RNLI lifeboat crew, but was later pronounced dead.
The discovery today followed the deployment of a remotely operated vehicle (ROV) off Blackrock this morning.
It gave search crews a clearer picture of the undersea conditions.
Conditions are being monitored hour-by-hour, in the hope that sea swells will ease to allow a more detailed examination of the site.
Describing the development as "hugely positive", the Chief Investigator with the Air Accident Investigation Unit said the discovery confirmed that the helicopter was located in the same place that the signal from the black box flight recorder.
Jurgen Whyte said finding the main part of the wreckage was an important step in the effort to locate the missing crew members.
He said the "absolute hope" was that they were still in the aircraft.
Mr Whyte said it remained to be seen if the ROV or dive crews would be best placed to access the wreckage in the coming days.
He identified the recovery of the crew as the main priority, followed by the recovery of the flight recorder.
Mr Whyte said information gleaned from that would help determine whether some, part or none of the wreckage needed to be recovered.
Garda Superintendent Tony Healy said the most up to date weather forecasting data was being used to plan the next phase of the operation.
He said the ROV was operating in the area of prime interest and could get right up to the wreckage.
While, there is no indication at this stage as to the extent of the damage to the helicopter, data from the ROV will be further analysed this evening and overnight to give a better impression of the likely cause of the crash.
Rescue 116 was providing cover for another Coast Guard helicopter on a mission at the time of the crash.
The families of the three missing Coast Guard personnel have been informed of developments. Some relatives visited the scene on board the LÉ Eithne this afternoon.
Sea conditions around Blackrock have complicated the search effort in recent days.
The LÉ Eithne is coordinating the sea-based search, assisted by the Naval Service and gardaí.
Meanwhile, Civil Defence teams have been carrying out drone searches of the coastline from Blacksod to Annagh Head.
Community effort to find missing crew
A local fisherman in Blacksod Bay said there is a huge community effort assisting in the search.
Speaking on RTÉ's Morning Ireland, Eamon Dixon said all the "community here really wants is for those missing crew to be recovered and they'll do just about anything that's asked of them to try and assist.
"It's an awful tragedy altogether. As well as that, they're the rescue services and they're the ones that come to save us - particularly at sea. We rely hugely on the helicoptor service. It's terribly, terribly sad."