Please note this section contains spoilers for the movie Serenity just in case you havn't seen it yet.
Miranda is a planet located in the Burnham Quadrant, on the very edge of the 'verse. At one time, it possessed an infrastructure as large and as thriving as anything to be found on a central planet. However, Miranda also apparently had a good deal of civil unrest, since the Alliance government decided to use the planet as a large-scale testbed for a chemical known as 'The Pax' which was intended to 'calm the population and weed out aggression.' For the vast majority of Miranda's unfortunate inhabitants, the Pax worked only too well, and millions of people 'stopped fighting...and then they stopped doing everything else.' Including staying alive.
For the few survivors, the consequences were even more horrific; they suffered a reaction to the Pax which was exactly opposite to the one intended-they became raging, homicidal maniacs who engaged in cannibalism, self-mutilation, and torture...in other words, Reavers.
After this, Miranda became a forgotten, inaccessible ghost planet until the secret of what happened there was accidentally gleaned from the minds of key Alliance Parliament members by a disturbed young psychic named River Tam.
During the events of Serenity Miranda is surrounded by a Reaver fleet. Malcolm and the crew of Serenity bypass this fleet by disguising their ship as a Reaver ship.