The effects of an underground nuclear test may vary according to factors including the depth and yield of the explosion, as well as the nature of the surrounding rock.[25] If the test is conducted at sufficient depth, the test is said to be contained, with no venting of gases or other contaminants to the environment.[25] In contrast, if the device is buried at insufficient depth ("underburied"), then rock may be expelled by the explosion, forming a crater surrounded by ejecta, and releasing high-pressure gases to the atmosphere (the resulting crater is usually conical in profile, circular, and may range between tens to hundreds of metres in diameter and depth[26]). One figure used in determining how deeply the device should be buried is the scaled depth of burial, or -burst.[25] This figure is calculated as the burial depth in metres divided by the cube root of the yield in kilotons. It is estimated that, in order to ensure containment, this figure should be greater than 100.[25][27]