Here's my version of the Abyss.
I wrote up the steps to create on my blog at http://terragen2.wordpress.com/2011/06/12/to-hell-and-back/
I'm still very much in learning mode. Comments welcome and encouraged.
Try making everything less uniformly lit, it doesn't look good, other than that it's a pretty good render
How do I change the uniformity of the light? Add additional light sources?
Nice cracked ground. I think your trees do need some work. And also (I'm not sure though) shouldn't this go in Image Sharing?
--Jahnu
That's where you are!
Try lowering the sun's elevation a little. How did you make it so uniform in the first place?