The plain of mud sphereteleports the player to a random location within the Goblin Cave. They can be purchased after the Land of the Goblinsquest, and are identical in appearance to Goblin village spheres. The sphere is consumed upon use and does not work beyond level 20 Wilderness. A skills necklace's teleport to the Fishing Guild can be used in place of the sphere, as it brings players just outside of the Goblin Cave.
Oldak, the cave goblin professor, will construct these spheres for the player if they provide him with 2 law runes and 1 molten glass (which can be noted).