Salvaging Vasa, 1959

Vasa or Wasa is a Swedish warship built between 1626 and 1628. The ship foundered after sailing about 1,300 m (1,400 yd) into its maiden voyage on 10 August 1628. The ship is one of Sweden's most popular tourist attractions and a widely recognized symbol of the Swedish "great power period".

The ship was built on the orders of the King of Sweden Gustavus Adolphus as part of the military expansion he initiated in a war with Poland-Lithuania (1621–1629).

Vasa sank in full view of a crowd of hundreds, if not thousands, of mostly ordinary Stockholmers who had come to see the great ship set sail. The crowd included foreign ambassadors, in effect spies of Gustavus Adolphus' allies and enemies, who also witnessed the catastrophe. Surviving crew members were questioned one by one about the handling of the ship at the time of the disaster. Was it rigged properly for the wind? Was the crew sober? Was the ballast properly stowed? Were the guns properly secured? However, no-one was prepared to take the blame. Crewmen and contractors formed two camps; each tried to blame the other, and everyone swore he had done his duty without fault and it was during the inquest that the details of the stability demonstration were revealed. "Why did you build the ship so narrow, so badly and without enough bottom that it capsized?" the prosecutor asked the shipwright Jacobsson. Jacobsson stated that he built the ship as directed by Henrik Hybertsson (long since dead and buried), who in turn had followed the specification approved by the king. Jacobsson had, in fact, widened the ship by 1 foot 5 inches (c. 42 cm) after taking over responsibility for the construction, but the construction of the ship was too far advanced to allow further widening.

In the end, no guilty party could be found. The answer Arendt de Groote gave when asked by the court why the ship sank was "Only God knows". Gustavus Adolphus had approved all measurements and armaments, and the ship was built according to the instructions and loaded with the number of guns specified. In the end, no-one was punished or found guilty for negligence, and the blame effectively fell on Henrik Hybertsson. The use of different measuring systems on either side of the vessel caused its mass to be distributed asymmetrically, heavier to port. During construction, both Swedish feet and Amsterdam feet were in use by different teams. Archaeologists have found four rulers used by the workmen who built the ship. Two were calibrated in Swedish feet, which had 12 inches, while the other two measured Amsterdam feet, which had 11 inches.

In the early 1950s, a number of possible recovery methods were proposed, including filling the ship with ping-pong balls and freezing it in a block of ice, but the method chosen was essentially the same one attempted immediately after the sinking. Divers spent two years digging six tunnels under the ship for steel cable slings, which were taken to a pair of lifting pontoons at the surface. The work under the ship was extremely dangerous, requiring the divers to cut tunnels through the clay with high-pressure water jets and suck up the resulting slurry with a dredge, all while working in total darkness with hundreds of tonnes of mud-filled ship overhead. A persisting risk was that the wreck could shift or settle deeper into the mud while a diver was working in a tunnel, trapping him underneath the wreckage. The almost vertical sections of the tunnels near the side of the hull could also potentially collapse and bury a diver inside. Despite the dangerous conditions, more than 1,300 dives were made in the salvage operation without any serious accidents.