I don't know if you already solved your problem with craters and holes. 
The friends already gave some instructions, I will give an idea, use a lot that in my works. 
 
Choose a mountain, turn her for the summit to be down aligning her/it to the ground, go up a little and later transform her/it in negative. 
Contain the mountain and the ground; 
You will have a hole... your size and depth will depend on the size and of the height of the mountain. 
Veja esss exemplo: 
http://www.renderosity.com/mod/gallery/index.php?image_id=1576506 
 
To prefer orders him/her the file Bryce. 
 
I hope has helped. 
Hugs