The Sky Pillar (Japanese: そらのはしら,Sora no wa Shira) is a huge tower located in Hoenn located at Route 131. Appearing in Pokémon Ruby, Sapphire, and Emerald, it is the only place in the game where the player can encounter (and catch) Rayquaza. Many other species of high-level Pokémon also inhabit it, such as Dusclops and Altaria. The Mach Bike is needed to climb to higher floors, and is inaccessible in Ruby and Sapphire, until the Elite Four are beaten.
In Emerald, it is accessible the second time after you have woken the Rayquaza to confront the Groudon and Kyogre in Sootopolis City. You do not need to challenge the Elite Four to catch it, unless however you are playing the Sapphire/Ruby games.
Rayquaza is always level 70 when you battle it, which is a major help with the Elite Four.