A type of nebula saturated with subnucleonic radiation deadly to most organic life, one of which forced Voyager's crew into stasis for a month while Seven of Nine ran the ship in One.
A Mutara class nebula is a huge cloud of gas whose interior is flooded with subnucleonic radiation. Exposure burns humanoid tissue within minutes, and the radiation also degrades starship systems over time.
In the Voyager episode One in 2374, the ship meets such a nebula, too wide to go around without adding more than a year to the journey. Crossing it will take about a month, so the crew are placed in stasis units while Seven of Nine, whose Borg physiology resists the radiation, and the holographic Doctor look after the ship.
The isolation is hard on a former drone used to the voices of the Collective. As systems fail, the Doctor is confined to sickbay and then goes offline, and Seven begins to hallucinate, among other things an alien intruder who taunts her. She brings the ship through by diverting power from life support to the engines, nearly at the cost of her own life.