Divers have found a sunken German U-boat submarine off the coast of Belgium, believed to have been struck by a mine a century ago during World War I.

The submarine is so well-preserved, officials say, that bodies of the crew are believed to still be on board.

“It’s quite amazing that we found something like this,” Western Flanders Governor Carl Decaluwe told the Associated Press. “The impact damage was at the front, but the submarine remains closed and there are 23 people still onboard.”

Belgian officials won’t release the exact location of the sub to keep looters from getting to it before they have a chance to excavate it.

Decaluwe says, too, that he’s contacted the Germans to hand over the remains of the soldiers aboard.

Germany was the first modern naval power to use submarines extensively, and its U-boats marauded the Atlantic in World War I, sinking some 5,000 ships over the course of the war.

At the outset of the war, military tacticians assumed that submarines would have limited range and would be used mostly to protect ports. The Germans proved them wrong, taking their submarines far out to sea and sinking both military and civilian merchant ships.

U-boats sunk a couple of big military warships in the North Sea during WWI, like the HMS Pathfinder and the HMS Formidable, forcing the British to keep their largest, most expensive war ships close to friendly ports.

When the Brits blockaded German ports, keeping supplies from reaching German shores, U-boats turned their guns on unarmed ships in retaliation to try to limit the flow of supplies to the Allies. The deliberate targeting of civilian ships — ships like the Lusitania and the Sussex that U-boats sank without warning — drew the U.S. into the war.