The Red Line is said to be 10,000 meters deep from ocean level to Fish-Man Island. At the same time, it is high enough above sea level to be considered impassable, and it is indestructible, meaning that it is almost impossible to pass over it or under it without using either of the entrances. Fisher Tiger, however, managed to climb up the entire Red Line with his bare hands to attack Mary Geoise. The whole of Germa 66 was also able to climb over the Red Line, from North Blue into East Blue, using its amphibious ships.
While the continent appears impossible 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: going up the waterways of Reverse Mountain (typically used by pirates to enter the Grand Line), gaining government permission from the holy government land of Mary Geoise, or submerging into an underwater passageway leading to Fish-Man Island, which is placed around a hole that links directly between Paradise and the New World.
At either side of the Grand Line, at the base of the Red Line, directly below Mary Geoise, is the Red Port, which has special lifts called Bondolas (ボンドラ,Bondora?) that reach Mary Geoise and allow those who were granted access to cross the Red Line.
On the exact opposite side of the world from Mary Geoise is Reverse Mountain, where water from the four Blues flows into the first half of the Grand Line. Twin Cape is built at the base of the mountain, similar to the two Red Port sections.