Black holes are a mystery. The 'ghosts,' if you will, of collapsed stars, they dot the universe and trap all matter that falls into them. In fact, the term 'back hole factory' has been used to refer to the plethora of these sites across the universe; they seem to be more abundant than planets with life on them.
Even light cannot escape black holes. The tremendous gravitational pull would crush just about anything that falls in. It is not known what lies on the 'other side' of them, but speculation has it that they may be wormholes into parallel universes.
This episode delineates the fascinating world of the great chasms, in an easy-to-follow but illuminative style. One of the best episodes of a series that really has no bad apples.