Poland's Navy said Thursday that it has identified a sunken shipwreck in the Baltic Sea as almost certainly being Nazi Germany's only aircraft carrier, the Graf Zeppelin — a find that promises to shed light on a 59-year-old mystery surrounding the ship's fate.

The Polish oil company Petrobaltic discovered the shipwreck earlier this month on the sea floor about 38 miles north of the northern port city of Gdansk.

Suspecting it could be the wreckage of the Graf Zeppelin, the Polish Navy sent out a hydrographic survey vessel on Tuesday, said Lt. Cmdr. Bartosz Zajda, a spokesman for the Polish Navy.

"We are 99 percent sure — even 99.9 percent — that these details point unambiguously to the Graf Zeppelin," Dariusz Beczek, the Navy commander of the vessel, the ORP Arctowski, said soon after returning to port Thursday morning after the two-day expedition.

During their time at sea, naval experts used a remote-controlled underwater robot and sonar photographic and video equipment to gather digital images of the 850-foot-long ship, Zajda said.

"The analyses of the sonar pictures and the comparison to historical documents show that it is the Graf Zeppelin," Zajda told The Associated Press.

Zajda said a number of characteristics of the shipwreck exactly matched those of the Graf Zeppelin, including the ship's measurements and a special device that lifted aircraft onto the launch deck from a lower deck.

The naval experts were still waiting to find the name "Graf Zeppelin" on one the ship's sides before declaring with absolute certainty that it is the German carrier, Zajda said.

The Graf Zeppelin was Germany's only aircraft carrier during World War II. It was launched on Dec. 8, 1938, but never saw action. After Germany's defeat in 1945, the Soviet Union took control of the ship, but it was last seen in 1947 and since then the ship's fate has been shrouded in mystery.

Navy researchers plan to continue to examine the material they gathered during their two days at sea, but the analysis of the shipwreck will then fall to historians and other researchers, Zajda said.

The Graf Zeppelin will almost certainly remain on the sea bed, he said.

"Technically it's impossible to pull it out of the water," Zajda said.