Game Development Reference
6. Make sure that shading is in GLSL, in the Scene Properies tab, or else change it. In
the Properies tab for the object data, check the Double side opion, if you don't
want to see the underwater environment. Normals are acive on the two sides of
the plane. If your whale dives, you can see the same texture from outside.
Normal maps simulate the impression of a detailed 3D surface by modifying
the shading as if the surface had being completely flat. Normals indicate the
direcion from the face, along a line.