Columbia, SC (AP) – Scientists studying the world’s first submarine to sink an enemy ship say the Confederate crew never released an emergency mechanism that could have helped them survive their mission.

The H.L. Hunley and its eight crewmembers disappeared in February 1864 in Charleston Harbor shortly after signaling it had placed explosives on the hull of a Union ship.

Ever since the Hunley was raised from the ocean floor in 2000, scientists have worked to determine why the sub never returned to the surface.

Their latest findings show the crew never released more than 1,000 pounds (454 kilograms) of blocks which were typically used to keep the submarine upright but also could have been dumped to enable a quick rise in an emergency.

The keel blocks go on display at the Hunley’s North Charleston museum on Saturday.