Dark nebula in the Crux constellation (Southern Cross).
It reduces the brightness of stars beyond it by 1–1.5 magnitude s. Easily visible against a starry background, it is probably about 550–600 light-years from Earth and 20–30 light-years in diameter. It figures in legends of peoples of the Southern Hemisphere and has been known to Europeans since с 1500. The Northern Coalsack, in the constellation Cygnus, is similar in nature and appearance but somewhat less obvious.