Ravines

Underground Ravines are a new natural structure coming to 1.8, announced by Notch on June 22, 2011. Ravines are tall, long structures, about 5-20 blocks in height and variably long. The width is barely a few blocks, usually no more than 5, sometimes making ravines hard to navigate from below because the ravine could go on the opposite corner of a block. Ravines can connect to caves, dungeons, and may have water/lava flowing down them. Ravines may also have small ledges along the top, possible to travel along. Ravines can be found at any level, and it is also 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)

A underground Ravine: