The crew of the Confederate submarine H.L. Hunley, the first combat submarine to sink an enemy ship, may have instantly killed themselves with their own weapon, according to a new study. This finding may have solved a mystery that has endured for more than 150 years about the fate of the sub.

The first and last combat mission of the Hunley took place during the Civil War on the night of Feb. 17, 1864. It attacked a steam-powered Union warship, the USS Housatonic, which was blockading the harbor entrance to Charleston, South Carolina.

The Hunley was a narrow, cigar-shaped submarine that measured 40 feet (12 meters) long and no more than 4 feet (1.2 m) wide. It was built from the wrought-iron boiler of a previous ship in 1863 and carried a crew of eight men and a powerful torpedo. [10 Epic Battles that Changed History]

The Hunley's torpedo delivered a blast from about 135 lbs. (61.2 kilograms) of explosive black powder below the waterline of the Housatonic's stern. The assault sank the Union ship in less than 5 minutes and killed five of its crewmembers. The rest escaped in lifeboats or were rescued by other members of the blockading force.

However, after the successful attack on the Housatonic, the Hunley failed to return to its base. The fate of the sub and its crew remained a mystery for more than 150 years.

Recovery efforts

In 1995, the Hunley was discovered about 985 feet (300 m) away from the watery grave of the Housatonic. The submarine was raised from the depths of Charleston Bay in 2000, and is undergoing study and conservation.

The discovery of the Hunley initially only deepened the mystery of its fate. Except for a hole in one conning tower and a small window that might have been broken, the vessel was remarkably intact, raising questions as to what killed everyone within.

In addition, the skeletal remains of the Hunley's crew were found seated at their respective stations, with no physical injuries or apparent attempts to escape. Moreover, the sub's bilge pumps, designed to pump water out of the sub, had not been used and its air hatch was closed. All the evidence suggested that the crew took absolutely no response to a flood or loss of air, said study lead author Rachel Lance, a biomechanist at Duke University in Durham, North Carolina.

A graphic reconstruction of the eight-man submarine H.L. Hunley as it appeared just before its encounter with the Union ship Housatonic, which it sunk. The barrel on the end of the 16-foot spar contains 135 pounds of black powder. (Image credit: Michael Crisafulli)

Now, researchers suggest that a deadly blast wave from the Hunley's own weapon may have killed its crew.

"Blast injuries are consistent with the way the remains were found inside the boat, as blast waves would not have left marks on the skeletons, and would not have provided the crew with the chance to try to escape," Lance told Live Science. "Blast waves are capable of inflicting lethal injuries on someone without ever physically moving them."

Torpedo tech

The Hunley's torpedo was not an underwater missile, but a copper keg of black powder held ahead of the submarine on a barbed pole, called a spar, that was about 16 feet (4.9 m) long. The sub rammed this spar into its target's hull and the bomb exploded, with the crew, at most, about 42 feet (12.8 m) from the blast. [Civil War Shipwreck: Photos of the USS Monitor]

To figure out how the Hunley's torpedo may have affected its own crew, the scientists conducted a series of experiments over the course of three years. This included repeatedly setting off pressurized-air blasts and black-powder explosions near a 6.5-foot-long (2 m) scale model of the Hunley, nicknamed the Tiny, that was fitted with sensors and floating in water.

The experiments often proved exasperating:"I was often frustrated with pressure gauges that wouldn't work, with black powder that got too wet to explode, or with weather that seemed to oscillate between freezing hurricane and blistering heat," Lance said. "These experiments were very difficult to conduct."

The findings from the experiments suggested that the Hunley's crew died instantly when the blast wave from the torpedo traveled through the soft tissue of their bodies, especially their lungs and brains.

"You have an instant fatality that leaves no marks on the skeletal remains," Lance said in a statement. "Unfortunately, the soft tissues that would show us what happened have decomposed in the past hundred years."

The kind of trauma the Hunley crew may have experienced is linked to a phenomenon that Lance called "the hot chocolate effect." This effect is linked to how vibrations such as shock waves travel at different speeds in water than they do in air — for instance, the shock wave from the Hunley blast would have traveled about 3,355 mph (5,400 km/h) in water but only about 760 mph (1,224 km/h) in the air, the researchers said.

"When you mix these speeds together in a frothy combination like the human lungs, or hot chocolate, it combines and it ends up making the energy go slower than it would in either one," Lance said in the statement.

This slowdown amplifies the tissue damage, Lance said. While a normal blast shock wave traveling in the air should last less than 10 milliseconds, Lance calculated that the Hunley crew's lungs were subjected to 60 milliseconds or more of trauma.

"That creates kind of a worst-case scenario for the lungs," Lance said in the statement. The force of the Hunley shock wave would have ripped apart the delicate structures of the lungs where the blood supply meets the air supply, filling the lungs with blood. This would have had at least an 85 percent chance of killing each member of the crew immediately, Lance calculated. It's also likely that these individuals suffered traumatic brain injuries from the blast, she added.

Blast waves

According to Lance, the way the torpedo's explosion may have killed the Hunley's crew was different from how traumatic blast injuries from modern-day improvised bombs kill soldiers in vehicles.

"In that case, there are shrapnel effects and effects from the damage to the vehicle that cause broken bones and other injuries," Lance said in the statement. "But the crew of the Hunley were protected by the hull. It was just the blast wave itself that propagated into the vessel, so their injuries would have been purely in the soft tissues, in the lungs and in the brain."

Still, it's possible for blast waves to travel through surfaces and still be powerful enough to kill, according to Lance.

"The Hunley is the first proven case study of lethal injuries from blast waves propagating through a solid surface," she said.

The designers of the Civil War-era torpedo may have recognized the dangers of getting too close to a blast in water. Lance's historical research found that the weapon's developers stayed hundreds of feet away from test blasts of explosives significantly smaller than the bomb the Hunley deployed. [Busted: 6 Civil War Myths]

"Blast travels really far underwater," Lance said in the statement. "If you're practicing 200 yards [182 m] away, and then you triple the size of your bomb and put it 16 feet [4.9 m] away, you have to be at least aware that there's a possibility of injury."

Modern warfare

Torpedoes were new technology at the start of the Civil War, Lance said.

"While their utility was immediately obvious, people were constantly concocting new designs and trigger mechanisms to try to improve them as the war progressed," Lance said. "The specific design used against the Housatonic, known as a Singer's torpedo, was one of the designs to emerge as the most successful. The early tests of submarines with torpedoes used smaller charges at a farther distance. The concerns were not that the blast would propagate through the hull; the science at the time was not nearly advanced enough to understand that that was possible. Rather, their concerns were that the torpedoes might damage the submarine itself."

The researchers think that after the attack, the Hunley then drifted out with the tides and slowly took on water before sinking. The sub's design was precarious — during development and testing, the Hunley had sunk twice, drowning 13 crewmen, including its namesake, the privateer Horace L. Hunley.

"I hope that, even though the mystery is now solved, people still visit and appreciate the Hunley for the incredible artifact that it is," Lance said.

Lance and her colleagues detailed their findings online Aug. 23 in the journal PLOS ONE. In addition, Lance is working on a book about the Hunley and the experiments that helped solve the mystery of its crew's fate.

Original article on Live Science.