Robert N. Dotzler, 24, died while free diving during an operation at the Navy base in Guam. He had apparently hyperventilated before the dive, lost consciousness underwater and drowned.

Petty Officer 2nd Class Taylor Gallant, 22, died Jan. 26, 2012, during another training operation while diving from a Canadian warship off the coast of North Carolina.

Reyher and Harris belonged to an underwater salvage unit. Gallant was an underwater bomb technician, the Navy said. Dotzler, a diver third class, was in the repair department of a submarine tender.

Such diving often is done in harsh conditions, in which divers are asked to find and work on things such as sunken ships and downed aircraft.

Perhaps the most famous such effort came in the salvage operations after the attack on Pearl Harbor during World War II. There, divers had to work in befouled waters, inside sunken battleships, amid dead bodies and unexploded bombs.

The Virginia Beach outfit that Reyher and Harris belonged to — Mobile Diving and Salvage Unit 2 (MDSU2) — in modern times has worked on the wreck of the space shuttle Challenger and has helped raise the massive turret of the sunken Civil War ironclad USS Monitor.

It used the super pond to train and evaluate its divers.

The unit’s commander, Runkle, was fired following the accident.

The Navy said that even before the tragedy it had been looking into the command climate at MDSU2, where it found poor morale, a lack of leadership and, later, safety issues.

In his essay, Runkle had been urging divers to be a little less can-do and a little more assertive in asking for more and better equipment.

The Army, for its part, closed the super pond indefinitely.

The body of water, which is 1,070 feet long and shaped like a frying pan, had been carved out of the east bank of Harford County’s Bush River in the early 1990s at the site of an old bomb-testing range.

Among many things, it was used to test the impact of underwater explosions on ship components.

The Army says the pond can handle a blast equal to 4,100 pounds of TNT — about the size of the truck bomb used to wreck the Oklahoma City federal building in 1995.

In December 2012, the pond was used to test a new device that was deployed from a helicopter to find and blow up underwater mines. The test had gone well. The tragedies came in the aftermath. The following account of the deaths comes from military reports.

A foot of visibility

The water temperature was a frigid 39 degrees with visibility less than a foot on Feb. 26 as Reyher and fellow diver Haamid Abdul-Mutakallim watched the scuba gear being set up.

The dive was supposed to have been done with Mark 16 re-breather units, which recycle air and allow more time underwater. But two of the four Mark 16s were broken, and three had to be working for a dive to be made.

The drill called for a dive to locate the carcass of an old helicopter. The water was pitch black at the bottom, where there was more than a foot of muck and a tangle of debris.

The divers would need flashlights and wire cutters.