A conspicuous dark nebula in the southern constellation Crux, easily visible to the naked eye as a dark patch against the brighter background of the Milky Way. It lies about 600 l.y. away and covers nearly 7° × 5° of sky. Its true diameter is about 60 l.y., and its mass is about 3500 solar masses. An open cluster, the Jewel Box, is visible next to the Coalsack, but is actually far more distant. The Northern Coalsack is another name for part of the Great Rift.

http://www.eso.org/public/images/b06/