Coalsack nebula

The Coalsack dark nebula (or simply the Coalsack) is the most prominent dark nebula in the skies. It can easily be seen with the naked eye as a dark patch silhouetted against the southern Milky Way. 
It was known pre-historically in the Southern Hemisphere and was observed by Vicente Yáñez Pinzón in 1499. The Coalsack is about 600 light years away from Earth, in the constellation Crux.