The Goo, also known as Contaminated Water,Moat,Toxic Water or simply Acid, is a common environmental hazard in both Portal and Portal 2. It is often found in Test Chambers, where it usually functions as a testing element, as well as in various maintenance areas. It also fills the very bottom of Test Shaft 09.
Goo is usually present within Test Chambers to add a hazardous element to the equation. Any contact with the Goo will result in continuous damage that would eventually kill a Test Subject within a few seconds. In Portal, the death appears as in the Half-Life series (i.e. the view of the bottom of the Goo pit, with the ability to look around); in Portal 2, the screen simply goes to green.
In Portal, the Goo appears as a mixture of red, green, and brown-colored liquid with a swirled appearance, often with smoke swirling around its presence. In Portal 2, the Goo appears to be mainly brown, though still with a swirled appearance.