The Gravity Well is another component of Stroggos' planetary defense system. A large device, it is capable of projecting a powerful cone of artificially high gravity into space, used by the Strogg to hold enemy ships in orbit above the planet. Its purpose in doing so is simple: Prevent enemy ships from escaping, neutralize their threat (once the ground invasion has been terminated) and harvest their occupants and/or technologies.
Destroying it is the main objective of the Quake 2 expansion pack, Ground Zero. During the events of Ground Zero, the Gravity Well traps all of the Human capital ships in orbit, which are then unable to escape or, because the Big Gun is still quite active, fight back. A small number of pods managed to successfully land on the planet before Gravity Well went online, simultaneously surviving the Big Gun's E.M.P. detonation. One of these pods belonged to Stepchild, the protagonist of the game. Stepchild would go on to destroy the Gravity Well, in the process defeating the Black Widow Guardian which was its protector.
Access to the Gravity Well can be gained by taking a Tram from the Ammo Depot to the Widow's Lair. Visitors and intruders must enter a web-like room surrounded by pillars. Intruders must defeat the Black Widow Guardian before being allowed to ride the elevator up to the Gravity Well.
The Gravity Well is a Black Core of Dark Matter, powered by twin reactors, which hold it stationary in mid-air with a strong yellow laser beam. There is a computer terminal connected to the Gravity Well, which is protected by two force fields. Controls for the force fields are found on the side of the terminal on the lower levels. The reactor is powered by two massive generators found in separate rooms.
The Black Core causes all light in the area to be blacked out every few moments, making walking around the core a hazardous affair, as the cooling pipes below will burn to death anybody unfortunate enough to fall in.
Stepchild arrives at the Munitions Plant and uses the Strogg's own production line to build an Antimatter Bomb. He then takes the bomb to the Widow's Lair (where the Gravity Well is located) and, in a desperate battle, defeats the Gravity Well's protector; the Black Widow Guardian. Stepchild then plants the bomb at the Gravity Well's computer terminal, ending the game.In the resulting end-game cutscene, Stepchild escapes back to the Research Hangar and takes off in the Prototype Strogg Fighter, which he secured after defeating the Carrier - a deadly aerial Strogg unit which hasn't been encountered before or since. Communicating with staff aboard the capital ship Deimos in orbit, it is revealed that the explosion of the Gravity Well generated a chain of explosions across Stroggos, causing complete the genocide of the Strogg race. This, however, does not fit in with the story of Quake 4, therefore players can assume that, while the chain reaction caused major damage on the planet, the vast bulk of the Strogg forces (and their most important facilities, including the Nexus) survived.