A new examination is being conducted on the wreck of the Baltic Sea passenger ferry M/S Estonia, which sank in 1994, claiming 852 lives.
The ship was traveling from the Estonian capital Tallinn to Stockholm on 28 September 1994 when it went down in the middle of the night off the coast of Finland.
Of the 989 people aboard, only 137 survived. It is considered Europe's worst maritime disaster since World War II.
A Swedish ship set sail on Thursday towards the site of the accident, and a second from Estonia was scheduled to depart from Tallinn in the evening.
The two ships should arrive at the scene around midnight (local time) and then begin preliminary surveys on Friday, explained Jonas Backstrand, chair of the Swedish Accident Investigation Board.
Neither the dead nor the wreckage were ever recovered and an international agreement declared the ruins a grave site in 1995, banning any disturbances.
A 1997 inquiry found that the ferry sank after its bow door was torn off in a storm.
But to this day there are doubts about the cause of the accident, and survivors and bereaved relatives have long demanded a fresh investigation.
The 1995 law that banned activity at the grave site was amended to allow further investigation into the shipwreck.
The new examination has been coordinated by Sweden, Estonia and Finland.
A memorial service was planned there on Friday before the underwater work begin, Backstrand said.
Sonar devices are to be used to examine the ship and the seabed, but there will be no human dives down to the wreck.
Preliminary assessments are to be made before more extensive investigations take place next spring, when the underwater visibility is expected to be better, Backstrand said.
The fresh investigation was prompted by a documentary released last year that raised questions about the cause of the sinking.
The filmmakers had used a diving robot to examine the ship and discovered a previously-unseen hole measuring several metres in the hull of the M/S Estonia.