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 to be grate of some sort. By looking carefully at the inventory in the PAX video, they appear to be iron bars.

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