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An ash cloud created by a volcano.
An ash cloud was a toxic cloud produced by an erupting volcano, during the time in which it expelled volcanic gases. (TAS: "The Jihad"; TNG: "Pen Pals", "All Good Things..."; VOY: "Basics, Part I", "Basics, Part II"; Star Trek Into Darkness)
On the planet Nibiru in the alternate reality of 2259, a volcano threatened to destroy the entire world as well as the pre-warp civilization inhabiting it – the Nibirans. Captain James T. Kirk decided to neutralize the volcanic threat by having Commander Spock, along with Lieutenants Hikaru Sulu and Nyota Uhura, place a cold fusion device inside the volatile volcano's chamber and thereby save the planet. To achieve this plan and not violate the Prime Directive, Spock, Sulu, and Uhura's shuttlecraft was concealed within the volcano's ash cloud. The ash cloud was eventually dissipated with the cold fusion device Spock, after much trouble, successfully detonated. (Star Trek Into Darkness)