The identity of the vessel — and how it was immobilized — remain a mystery. Mr. Termote is determined to discover the truth, and he is focusing on several clues.

Two of the vessel’s torpedo tubes were destroyed and found lying a short distance away, but a lower tube remains intact. Two of the hatches are still closed, but one was open; Mr. Termote said it had allowed him to see that the submarine was filled to the ceiling with sand. That suggested that the remains of the commander and his crew — typically 22 in such a U-boat — would be buried within.

“We are certain that the crew is dead and still inside, likely buried in the sand, though it is possible that some escaped,” he said. “It takes about six months to a year for bodies to decompose underwater, so by now they would be reduced to bones. It is a total mystery why the submarine is still there. There are no witness accounts from that time of a sinking.”

After carefully examining the evidence, he said the damage to the vessel suggested that it had probably escaped after being bombed by British forces and was on its way back to the German naval base in Bruges when it hit a British mine and sank.

Another hypothesis was that it was sunk by a depth charge, an explosives-filled canister dropped from the stern of a ship to destroy a submarine.

German U-boats were lethally effective. During World War I, the 93 U-boats stationed at Belgian ports sank more than 2,550 Allied ships, but their relatively slow speed made them vulnerable to mines and depth charges. At least 70 U-boats never returned to Belgium, and about 1,200 German sailors were confirmed dead, Mr. Termote said.

The murderous power of U-boats entered the global consciousness on May 7, 1915, when a German U-boat torpedoed the ocean liner Lusitania off the Irish coast, killing 1,198 people — including more than 100 Americans. The sinking was used to demonstrate German barbarity, and it played a pivotal role in galvanizing the United States to enter the war.