Creating Complex Natural Materials in Cycles - Blender 2.6 Cycles: Materials and Textures - page 165

And here we are so far:
What is missing now is the underwater deep atmospheric perspective effect. There
are several ways to obtain this, for example, by compositing a Mist pass rendered in
Blender Internal, but we are going to do it with a node group assigned to each one
of the different materials:
1. Add a Camera Data node (press Shift + A and go to Input | Camera
Data ), a Math node (press Shift + A and go to Convertor | Math ), an
Emission node (press Shift + A and Shader | Emission ), and a Mix
Shader node (press Shift + A and go to Shader | Mix Shader ).
2. Connect the View Z Depth output of the Camera Data node to the first
Value input of the Math node. Set the Math node operation to Multiply
and the second Value to 0.030 , check the Clamp option. Connect the
Multiply node output to the Fac input socket of the Mix Shader node.
3. Connect the Emission output to the second Shader input of the Mix
Shader node and set the color to R 0.040 , G 0.117 , B 0.124 .