The quality of graphics can be controlled via the Options → Advanced page. By default this is in auto mode, but you may wish to apply some manual control.
Press the Graphics Settings button to toggle it from Auto Detect to Set Manually. You can now force a particular graphics quality. The lowest graphics quality setting is Retro Line Mode, a retro mode where the tunnels will be displayed as a mesh of lines. Highest will set the display of the tunnel and other objects to maximum quality. You can also select Custom quality settings.
Custom ModeCustom graphics quality allows extra control over how the tunnel and other objects are displayed.
Texture Quality will affect how long it takes to load a new tunnel. If you notice pixelation on the tunnel wall you may wish to increase the texture quality.