Non-corporeal life forms residing from a planet completely submerged by their form. They are able to take a liquid shape and transform from it to gas at will. In it's gaseous form, the life form can enter a human and take control, similar to a symbiote. Colonel Maybourne assured SG-1 the species' intentions were not peaceful.
Russian Stargate personnel took a sample of the liquid substance back to Russia, noting that it was generating an abnormal amount of energy in the form of heat. Apparently someone accessed the stored container and the fluid overtook his body, among others. The rest of the personnel shot each other, and in a last-ditch attempt to halt the mayhem the facility was gassed.
It was eventually discovered the lifeforms only wished to return to their realm, and re-deposited Major Carter, Dr. Jackson and Dr. Markov, who tried to explore their home, back to Earth. O'Neill suggested they had just exchanged hostages.
Watergate - O'Neill and Teal'c discover Colonel Maybourne has been possessed by a gaseous lifeform, one that leaps into Teal'c to return through the Stargate to its home.