Tetraburnium was a metallic alloy capable of withstanding extreme amounts of heat and pressure.
In 2375, at Seven of Nine's suggestion, the Delta Flyer was constructed of tetraburnium alloys, allowing it to enter the atmosphere of a class 6 gas giant and retrieve a multi-spatial probe. A Malon shuttlecraft, built for the same purpose, also had a tetraburnium hull. (VOY: "Extreme Risk")