Nested isolines illustrate the use of black fog to do depth-cueing. That cueing makes distance shapes darker and helps to declutter scenes, particularly when drawing wireframe shapes like this isoline shape.
Vrml97ToX3dNist, http://ovrt.nist.gov/v2_x3d.html
X3D-Edit, https://www.web3d.org/x3d/tools/X3D-Edit