Confederate Sub Still a Mystery: Why Did It Sink? Scientists can't say what caused HL Hunley to go under By Neal Colgrass, Newser Staff Posted Feb 18, 2014 5:15 PM CST Updated Feb 22, 2014 7:34 AM CST 45 comments Comments The Confederate submarine HL Hunley sits in a conservation tank after a steel truss that had surrounded it was removed in this Jan. 12, 2012, file photo, taken at a lab in North Charleston, SC. (AP Photo/Bruce Smith, File) (Newser) – A revered Confederate submarine was recently commemorated by military reenactors in Charleston, South Carolina, but a key mystery still hung in the air, the Christian Science Monitor reports. The HL Hunley made history on February 17, 1864, when it became the first sub to ever sink an enemy ship—in this case, the Union's Housatonic, which was part of a blockade on a moonlit night around the key Southern port of Charleston. The submarine lodged an explosive in the Housatonic's wooden side that went off, and the Hunley's commander signaled rebels on the shore with a blue magnesium light for victory. But then something went very wrong, the New York Times reports. The Hunley proceeded to sink, killing its 8-man crew and sinking to the bottom of Charleston's harbor. The submarine had encountered disaster before, killing 17 crewmembers in two training accidents, History reports, but this time it stayed down until being found in 1995 and raised and returned to harbor in 2000. The discovery team analyzed the vessel, marveling at its engineering and graceful lines. But what on Earth caused it to sink? One theory notes that while the sub's torpedo sat at the end of a 17-foot spar, the vessel may have been only 20 feet away when the explosive went off, sending the Hunley and its unconscious crew to the bottom.