In the world of non-mainstream music, post metal is a relatively new and unknown genre. Perhaps hard to distinguish from post rock, both can be described as new age music with rock instruments. While often starting with soft ambient melodies, it can very quickly develop into hard metal driven progressions.
The video here features one of the premier bands in post metal, Russian Circles. Other notable bands include Neurosis and Isis which have been around for a while but not as long as other genres of music.