Green Goo is supposedly to be an extraterrestrial substance from outer space alien culture, it is found and kept in guard by the US secret divisions of weapon development, in Area 69.The Green Goo's origin is not known yet, but according to it is texture and model files, it must be either an Alien blood or a UFO's fuel. The Green Goo took its name from its appearance, which is a slimy green substance stored in a transparent container.
The Green Goo is featured in one of the Truth's missions where CJ must steal it from a military convoy train. By noticing the huge amount of guards that make sure it reach its destination, it can be easy to determine that it has a major importance for the US government.
According to some players, the Green Goo emits a green laser at certain time of days and emits a sharp noise. The laser claim is not approved yet, but it does emit a sharp noise.