Ravines

Underground Ravines are a new natural structure coming to 1.8, announced by Notch on June 22, 2011. It is possible that ravines may scarcely appear on the world surface, forming canyons. Also look in the lit up version of the picture, in the upper-right corner it appears as a grate, or gate of some sort.

During the QA session at PAX, a ravine was captured in fairly good detail on Fdoke's Livestream. (at 24:00 in)