The Coalsack, also known as the Coalsack Dark Nebula, is the finest example of a dark nebula in the sky. It lies in the southern constellation Crux, in the galactic plane, about 550 light-years away, and is readily visible to the naked eye against the background of the Milky Way (position: RA 12h 50m, Dec -62° 30'). The Coalsack spans some 60 light-years (7° × 5° of the sky), and has an estimated mass of about 3,500 times that of the Sun.
The Coalsack in science fiction
In the Star Trek universe, the starship Enterprise was commandeered in AD 2269 by Bele of the planet Cheron, who attempted to divert the ship from its position near Ariannus. Ensign Chekov remarked that the course would have taken them directly toward the Coalsack (TOS: "Let That Be Your Last Battlefield").
In Larry Niven and Jerry Pournelle's novel The Mote in God's Eye, the three-armed aliens known as the Moties were first first contacted in AD 3017 in the region of the Coalsack.