Big Blue is a fictional planet found in the F-Zero series that is mainly (about 99%) covered by water. The buildings and oftentimes F-Zero tracks have obviously been built above the water, though sometimes you'll see both the buildings and track go through underwater tunnels and what not.
Because of the great difficulty of building anything on the planet, there are few buildings found. Still, it's beauty makes it a popular destination - Especially for F-Zero grand prix races.
However, recently the planet has become more popular and now features hotels, industrial buildings, and so on, as well as the always popular grand prix tracks. Life can apparently be sustained underwater, as colossal fish can be swimming under there, even while racing in the transparent tunnels.
Super Smash Bros. series
A Big Blue stage was featured in Super Smash Bros. Melee for the GameCube. It was unlockable and naturally (for an F-Zero stage) was a scrolling stage. Vast amounts of water could be seen in the background, though players were required to pay attention to the action going on while fighting.
Players would not only have to fight their opponents on this stage, but would also have to jump to various vehicles that were racing at the time as not to get sent off the screen.