On March 18, 1945, Lt. j.g. David L. Mandt took off from Maryland’s Patuxent River Naval Air Station to test out the guns on an experimental fighter plane, the XF8F-1 Bearcat.

Mandt was a veteran aircraft carrier pilot who had flown in battle off the deck of the USS Bunker Hill in the Pacific. He had shot down Japanese planes. He had been on numerous raids on enemy forces. His plane had been riddled in combat.

With World War II turning against Japan, he was back stateside trying out the Navy’s latest fighter. It was the first of its kind, a propeller-driven hot rod with a 2,100-horsepower engine. In photographs, it appeared with the word “TEST” emblazoned on its side.

Mandt, 23, a native of Detroit, took off at 2:15 p.m. and flew out over the glassy surface of the bay.

His plane never returned.

The water was unusually clear in the Chesapeake Bay the day diver Dan Lynberg first descended to the bottom to examine the object that had turned up on his sonar survey.

A member of the Institute of Maritime History, a volunteer group of underwater archaeologists, he had gotten the sonar “hit” at a spot east of the air station during a routine survey, he said. He was now going 80 feet down from a dive boat to investigate.

Skilled in “blackwater” diving, where visibility is poor to zero, he was amazed at the day’s clarity. “Got a really good view” of the object, he said.

It was covered in sea growth, fishing debris and a layer of silt, and it seemed to almost be part of the bottom of the bay.

He could tell it was a single-engine airplane, compact and rugged-looking. He did not recognize the model, “but I could tell by the structure and the wings that it was either a military fighter or aerobatic [airplane], just by the strength that was built into the wings.”

He noticed the engine was torn off the front. The bubble canopy had slid open, and the cockpit was piled almost to the brim with sediment.

But where was the pilot? Did he get out of the plane? Could his remains or effects still be in the cockpit?

“Do not know,” Lynberg said.

He tried taking pictures, but they did not turn out. He surfaced and had already noted the location.

That was about 2010, he said.

About three years ago, he said, the Naval History and Heritage Command asked institute volunteers to conduct a search for lost aircraft in the bay near the air station. The Navy had begun a systematic search for such planes. One of them was Mandt’s Bearcat.