Twenty people died on Saturday in Russia's worst naval accident since a nuclear submarine sank and killed all 118 crew members in 2000. While it was being field tested in the Sea of Japan over the weekend, the submarine’s automatic firefighting system was unexpectedly triggered, causing a fire suppressant(抑制剤)gas that displaces breathable oxygen to be released. Most of the dead were civilian members of the shipyard crew, and some speculate that they may have not known how to react when the gas was released. It is unclear why the portable breathing gear typically issued to Russian submarine crews did not save them, and an investigation into the incident is underway.