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Magrathea is an ancient planet located in orbit around the twin suns Soulianis and Rahm in the heart of the Dark Nebula of Soulianis and Rahm (a.k.a. the Horsehead Nebula).

It was the home of a new form of specialist industry: custom-made, luxury planet building. Hyperspatial engineers sucked matter through white holes in space to form dream planets - gold planets, square planets, glass planets, platinum planets, soft rubber planets with lots of earthquakes, planets covered with fish - all lovingly made to meet the exacting standards that the Galaxy's richest men naturally came to expect.

It was the Magratheans who constructed the planet-sized computer, Earth for a race of hyperintelligent pan-dimensional beings (whose intrusions into our dimension appeared as white mice), and designed by Deep Thought; to determine the Ultimate Question of Life, the Universe, and Everything; which is in turn required to understand the Answer to Life, the Universe, and Everything.

Unfortunately, the venture was so successful that Magrathea soon became the richest planet of all time and the rest of the Galaxy was reduced to abject poverty. The Magratheans went into hibernation, awaiting an economic recovery that could afford their services once more. Mostly everyone except Slartibartfast is seen to be in hibernation.

Magrathea itself disappeared and its memory soon passed into the obscurity of legend.

In these enlightened days, of course, no one believes a word of it.

After the Vogons destroyed the supercomputer Earth (5 minutes before the Time of Readout), the mice decided to re-do the experiment and the Magratheans were contracted to build the Earth Mk II (upon which Slartibartfast decided to edge Africa with fjords); but they decided that since they had a last-generation sample of Human brain tissue (i.e. Arthur Dent's brain), they would cancel it and extract the Question from Arthur's brain, a process that involved pureeing said brain. Luckily, Arthur managed to kill the mice and escape, only to be displaced to the Restaurant at the End of the Universe which would eventually be built on the remains of Magrathea. However, since Arthur was actually descended from the Golgafrinchans, who had crashed on Earth two milion years ago and killed off the native Australopithecines, the Question he had (Probably What do you get when you multiply six by nine? as drawn from a Scrabble® tile bag by Arthur) was almost certainly wrong.

A decade later, when the Guide MkII and the Vogons destroyed the Replacement Earth which had popped in from another dimension, using the Grebulons as patsies, Zaphod Beeblebrox had the Magratheans construct a planetoid with an Earthlike environment and sold space on it to Hillman Hunter (who named it after his grandmother Nano) and to Aseed Preflux (Hunter's doppelgänger from yet another alternate Earth)...