The Russian stargate program discovered an unnamed planet with the ruins of an ancient extinct civilization, now completely under water. Incredibly, a sample of this water spontaneously emitted energy in the form of heat. On the planet, the combination of this energy and a strong signal from a reconnaissance drone aimed at the event horizon had maintained the wormhole connection beyond the normal 38-minute maximum and wouldn't allow the wormhole from Earth to shut down.
The heat being emitted was due to countless microscopic intelligent life-forms in the water that are capable of entering a human body in the form of inhaled vapor, and taking control of the body or leaving it again at will. When a sample of the water had been brought back to Earth for study by the Russians, these living organisms had taken possession of the Russian personnel, forcing them to redial the gate so that they could be returned home. Meanwhile, the water life-forms on the planet seized the exploratory Russian mini-submarine in which Carter, Daniel, and Svetlana Markov were traveling, and returned the humans to Earth through the stargate in a sort of hostage exchange.
Cross Reference: Hazmat, Svetlana Markov, Water Planet
Episode Reference: Watergate