From the Super Mario Wiki
Big Blue is a race track of the F-Zero Grand Prix. It has gone down as probably the second most famous track in F-Zero history, losing only to Mute City. Actually, Big Blue is the name given to the planet where this race happens, so called because 99% of its surface is an immense ocean. It also appears as an unlockable stage in Super Smash Bros. Melee, and returns in Super Smash Bros. Brawl.
The stage is side-scrolling, and the fight takes place on top of the F-Zero gliders. Players have to be cautious not to hit the ground, as it scrolls incredibly fast, giving players only a split-second to jump off before they are scrolled off of the screen and KOed. There are also numerous platforms that pass by, so players can head up on them to rest a bit. However, it is easier to get attacked by other players when on the platforms. The fight starts on the Falcon Flier, the personal spacecraft of Captain Falcon.
It is unlocked by playing 150 Vs Matches in total. The player would even be rewarded the F-Zero Racers Trophy after unlocking the Stage.
There is an alternate music track to Big Blue. It is the theme for Mach Rider, another futuristic racing game for the NES. However, that music track has been moved to Port Town Aero Dive in Super Smash Bros. Brawl.