The Vasa Museum is partly built on top of a drydock (Galärvarvsdockan).
The front part of the dock was not covered and can be found at the entrance to the museum. In the summer, it is filled with water, and there you can see a copy of the Zetterström nozzle, a tool that the divers used when they dug tunnels under the Vasa's hull, during the salvage of the ship.
When the museum was built, one end of the building, the one by the water, and the opening of the dock, were left open. In September 1988, preparations to move Vasa from Wasavarvet (the old museum building) began. The ship was wrapped in a protective shell, and resting on a pontoon, she was moved by sea to the new museum in December of the same year.
At the front, the ship was placed in the dock. Slowly they began to let the water out of the dry dock and then build the wall that was missing. The pontoon Vasa was towed into the museum on, is today the floor of the museum's lowest level.
On the website you can see a photograph of what it looked like when the Vasa was towed into its new museum; http://www.vasamuseet.se/sv/Skeppet/Vasa-idag/