A funnel-shaped hole sucking its texture to the middle.
The effect is accomplished like the tunnel effect but this time with a 15 x 15 texture and static lookup table (wormhole.gif or calculated array).
The texture is shifted and mapped to the static lookup table. One of the most famous demos that has a worm effect is Unreal by future crew.
Press  -  to load new textures Press arrow keys to change direction [s] will stop animation
Source code: Wormhole