Sandworm Juice is a item dropped by the sandworm mob, a very powerful mob that lives in the desert, under the sand, only coming up to attack players.
As this is very difficult to obtain, it is used to craft what could be the most important structure in the game: the Emerald Machine; and the most powerful arrow: the Dragon Arrow, capable of killing all boss mobs in only a few (2-5) hits.
Forest Mode, previously known as Legacy Mode, represents the original version of starve.io, with a few changes, such as the addition of the Emerald Machine. However, there is only forest in Forest Mode, which lacks the sandworms and emeralds that give the necessary resources for the creation of the Emerald Machine. Because of this, all players spawn with 1 sandworm juice in Forest Mode, and there are a few emerald resource nodes scattered around the map.