Ravines

Ravines are a new natural structure, introduced in Beta 1.8. 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 also scarcely appear on the world surface, forming canyons. Also, in the upper-right corner of the lit up version of the picture, there is what appears to be a grate of some sort. This could be another natural occurrence of iron bars.

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

An underground Ravine: