Canyon was once an uninhabitable Mars-like world known as Warhead. It is the second of seven planets around p Eridani A, 22 light-years from Earth.
It was used as a military outpost by the Kzinti (who conquered it around 1850 due to the fact it was close to many non-occupied Pierin worlds), until the planet was hit by a weapon called the "Wunderland Treatymaker" during the Third Man-Kzin War. The attack tore a long, narrow, kilometers-deep crater into the crust approximately the size of the Baja Peninsula. The air and moisture in the thin atmosphere gathered at the bottom of this artificial canyon, creating a breathable environment, complete with a sea at the bottom. Conquered by Humans around 2500 in the Third Man-Kzin War. The planet was then renamed for the canyon-crater, and settled by humans in a huge city running up the crater wall.
Archaic (hyper-aggressive and intractable) Kzinti were entombed in stasis field shells during the attack and are still beneath the lava, and someday, somebody will have to deal with them.