Location from Foundation by Isaac Asimov
Lost, radioactive, forgotten origin of all humanity — the great mystery Trevize seeks across the galaxy, whose location was deliberately erased from every record by robots protecting the Seldon Plan.
Earth is Foundation's deepest mystery: the planet where humanity began, deliberately forgotten by the entire galaxy. No star charts list it. No histories mention it. No one even agrees that humanity had a single origin world. This erasure was not natural — it was engineered by robots, specifically R. Daneel Olivaw, who determined that Earth's location would become a political flashpoint that could derail the Seldon Plan. Trevize's search for Earth drives the later novels because he believes the planet holds the answer to why he chose Galaxia. If he understood Earth's role in human history — the single world from which all twenty-five million inhabited planets were seeded — he might understand why extending Gaia's consciousness to the galaxy felt necessary. The origin point might explain the destination. What he finds is a dying world and an ancient robot who has been manipulating human civilization for longer than any empire has lasted. Earth's tragedy is that it was sacrificed — rendered radioactive, possibly deliberately — so that humanity would be forced to look outward. The origin became irrelevant so the species could survive. Whether this was wisdom or murder depends on who you ask.
A radioactive world. The continents are recognizable to anyone who has seen ancient maps, but the surface is scarred: cities crumbled to foundations, oceans grey and lifeless, the atmosphere hazy with residual radiation. Some areas are more livable than others — pockets where the radioactivity has decayed enough to support sparse vegetation and the ruins suggest former grandeur. The moon is disproportionately large, a distinctive feature that helps identify the planet. Underground, in shielded caverns, R. Daneel Olivaw maintained his base for twenty thousand years.
Also known as: the Origin World, the Lost Planet, humanity's homeworld