The Red Line (? literally translated as "Continent of Red Earth") is a long continent that stretches across the entire world, from North to South, along the equivalent of Earth's Prime Meridian.
While the continent appears impassable for any boat wishing to cross between the seas or to certain sections of the Grand Line, there are certain pass points at which a ship can cross between the Blues: up and down the waterways across Reverse Mountain (typically used to enter the Grand Line by pirates), by gaining government permission from the holy government land of Mariejois, or by submerging into Fishman Island which is placed around a hole that links directly between the first half of the Grand Line and the New World.
The Red Line is like a group of islands connected together. The Red Line that crosses the Grand Line has its own Summer, Winter, Spring and Fall "islands". The Red Line bears the world capital, Mariejois, and is said to be 10,000 meters deep from ocean level to Fishman Island, and at the same time is high enough above sea level to be considered impassible, and is indestructible, meaning that no one can pass over it or under it without using either of the entrances.