Two tons of cast-iron keel blocks steadied the Confederate submarine H.L. Hunley underwater.

The crew could have freed the eight separate blocks from the bottom with an emergency release mechanism, but they were never unlatched.

That finding deepens the mystery of just why the eight men inside never made it back.

In another disclosure of the sub's mechanics, scientists studying the Hunley said Wednesday evidence shows the crew did not think a quick release of the outside bottom weights would have helped whatever situation the vessel was in after sinking the Union blockade ship Housatonic.

"As a diver, your first instinct if you're in trouble is to get to the surface by releasing your weight belt, and it’s part of your training," said Johanna Rivera, a Clemson University conservator working on the project.

"The keel blocks serve as the same purpose, so it appears there was no sense of panic (among the crew)," she said. The finding "is an extra layer of complexity as to what really happened."

The hand-cranked 40-foot-long Civil War craft became the world’s first successful attack sub by sinking the Housatonic off Sullivan’s Island on Feb. 17, 1864, by ramming a powder explosive into its hull. While the crew signaled they planned to return by reportedly flashing a blue lantern light toward shore, they never appeared.

The sub and the remains inside were recovered 4 miles offshore in 2000. The crew was later buried at Magnolia Cemetery in Charleston.

Still undetermined is what caused the vessel to perish. One of the more popular theories is the sub got stuck on the bottom while the crew waited for the tide to turn. The weight of the blocks would have held it there.

But the heaviest of the blocks had been designed to be quickly released with a turnkey in an emergency. 

Other theories are that the crew might have been injured in the explosive blast or became incapacitated after suffering a loss of oxygen.

Since the recovery, the sub has been undergoing conservation work at the Clemson University-run Warren Lasch Conservation Center in North Charleston, on the grounds of the former Naval Base and Shipyard. It's open for tours on weekends.

The recently conserved keel blocks are now be part of the display.

Sign up for our daily newsletter

Get the best of The Post and Courier, handpicked and delivered to your inbox every morning.

Reach Bo Petersen Reporter at Facebook, @bopete on Twitter or 1-843-937-5744.

Science and environment reporter. Author of Washing Our Hands in the Clouds.