Sweden and Estonian authorities have launched a fresh inspection into the shipwreck of a Baltic Sea ferry disaster in 1994.
The sinking of the MS Estonia was one of the deadliest peacetime maritime disasters in European waters, killing 852 people.
An international investigation in 1997 concluded that a fault in the ship's retractable bow door had flooded the car deck, causing the 155-metre long vessel to fill with water and capsize.
But a 2020 documentary has since cast doubt on the official version of events, broadcasting footage of a previously unknown four-metre hole in the ferry's hull.
Survivors and relatives of the victims have long demanded that the inquiries be reopened.
The new inspection will use an array of different sonar devices to examine the position of the ship on the seabed, authorities said.
Jonas Bäckstrand, chairman of investigations at the Swedish Maritime Accident Investigation Bureau (SHK), said on Thursday: "The Estonian icebreaker EVA 316 and the Swedish research vessel ELECTRA have left their respective ports."
"[They] are expected to converge before midnight for joint exercises to ensure the safety of the equipment." Dives in international waters will start on Friday, he added.
The MS Estonia lies 85 metres underwater in the Baltic Sea, and many of the victims' bodies are still believed to be there.
Since 1995, Sweden, Finland, and Estonia have banned any exploration of the wreck under an international agreement. But late last year, Sweden had said it was ready to lift the ban and allow new inspections to take place.
Last October, Estonian Prime Minister Jüri Ratas also called for a new investigation into the MS Estonia tragedy "as soon as possible". The new probe could be completed as early as spring 2022.
Before the dives began, and as a mark of respect for the victims of the disaster, a religious ceremony involving bishops was held onboard an Estonian coastguard vessel near the location of the wreck.
"It is very special to be here, knowing that this is the final resting place of more than 700 people," Bäckstrand told Euronews.