The Well World is a fictional planet in Jack L. Chalker's "Well of Souls" and "Watchers at the Well" series of novels.
The Well World was constructed by an ancient alien species known as the Markovians who predated the existence of the current Universe. They were the first and only intelligent species to arise in the original universe and developed immense levels of technology that allowed them to manipulate reality on a basic level. They constructed layers of machinery miles beneath the surfaces of their colony worlds that responded to telepathic commands (in a manner very similar to the Krell Machine in Forbidden Planet). Their localized omnipotence did not lead to self-destruction (as it did for the Krell), but eventually the Markovians developed a sense of ennui and incompleteness that they were unable to fully understand or resolve. The Well World was eventually constructed as a means of trying to overcome whatever flaw in themselves was stopping them.
The Well World is a computer roughly the size of Earth with a crust of normal rock, water and air over the surface making it resemble a normal planet. The computer is responsible for "simulating" an entire new universe superimposed over the old Markovian one. The surface is divided up into 1560 large hexagonal regions (780 per hemisphere) each with an independent and often dramatically different climate. Each is home to a new species of intelligent life developed by the Markovians, and supports a maximum tech level enforced by the computer (called the Well of Souls). Devices above the allowed tech level simply fail to function in low-tech hexes. The southern hemisphere is devoted to relatively Terrestrial environments with atmospheres high in oxygen and carbon-based life (or, in some cases, life forms compatible with carbon-based species). The northern hemisphere is devoted to more exotic and variable environments (often with non-carbon biology), with an enormous wall at the equator keeping them separate. Two large circular regions known as the Northern and Southern Zones cap each pole. Each Zone contains gates for entering the Well World from elsewhere in the universe, embassies for each of the hexes in their hemispheres and gates to those hexes, and gates for travel to the opposite Zone. Outgoing travel from a Zone is limited to one's home hex or the opposite Zone. Hexes bordering the equatorial and Zone walls are divided in half and joined with neighboring hex-segments to form wing-shaped regions. Trenches known as 'Avenues' form the borders between the equatorial hexes, and provide entry to the Well's controls.
The Markovians travelled en masse to the Well World and used the computer to translate themselves into members of the various species living on it. After residing in that species' hex until the species' viability had been confirmed, they would be teleported out to colonize an appropriate planet in the new universe. In this manner the Markovians started their technological and social development over from scratch. By using thousands of different approaches to existence, they hoped that once they reacquired their old level of technology one of their child species would know what to do with it. Humanity was one such created species.
The Markovians have all passed through the Well in this manner (with one or two possible exceptions) but the ancient Markovian colony worlds scattered throughout the new universe still sometimes had functional teleportation machinery that would inadvertently snare individuals and send them to the Well World. These individuals would be automatically transformed into randomly selected species on the well just like the Markovians were, but without Markovians running the operation new species were no longer being created or sent out to other planets.
In later books a human scientist named Zinder developed a technology that allowed a human-built AI named Obie to remotely interface with the Well of Souls and change local regions of reality without initially realizing that this is how the reality-manipulations were being done. Zinder and Obie were ultimately lost but human governments later reconstructed part of his research and developed the Zinder Nullifier, a weapon that simply "blanked" the region of space it was targeted at. The Nullifiers were a vital weapon in winning a war of species survival against extragalactic invaders, but the 'blanking' was creating uncontrolled areas of space, in effect, shorting the Well's controls. When this reached a certain size, it became too much for the Well's basic error handling procedures to deal with and became self perpetuating, like a minor tear slowly growing under stress to a fabric. An expanding region of the universe began going blank and the Well of Souls ultimately had to be rebooted to prevent it from being irreversibly damaged, causing the Universe to end and be recreated.