Externally, Daleks resembled human-sized peppershakers, with a single mechanical eyestalk in a rotating dome, a gunstick and a manipulator arm. The casings were made of a material that was also called dalekanium. The lower shell was covered with many hemispherical protrusions, seen as a self-destruct system. The Dalek's eyepiece was its most vulnerable spot, and impairing its vision often led to its main weapon being fired indiscriminately. The Dalek casing also functioned as a fully-sealed environment suit, allowing travel through the vacuum of space or underwater without the need for additional life-support equipment.