The entrance to the city is a 90-degree angle, as confirmed in the episode of the same name. Bikini Bottom cannot be seen from Rock Bottom due to the 90-degree angle on both sides and due to no sunlight reaching it.
The road between Bikini Bottom and Rock Bottom is so perfectly vertical that it can only be accessed or departed by a bus or balloon and also has a 90-degree angle when entering and exiting the city to go back into Bikini Bottom.
