- For Bonus Stages in general, see here.
Hidden in the game are two special doors. Unlock the doors, and for a limited time you will enter a special Bonus stage, where you grab rings for extra points. How do you unlock the doors? Therein lies the challenge... though you might receive a hint once you've defeated Dr. Robotnik!
— US instruction booklet, Sonic Labyrinth
The Bonus Stage in Sonic Labyrinth is a small hidden level where the player can get the final Chaos Emerald; the other four are dropped by each boss in the game upon defeat, and the fifth is dropped by Dr. Robotnik after the battle with the Smiley Bomb.
To access the Bonus Stage, the player must find the hidden door to it in Zone 2-3 (Labyrinth of the Sea), which is located on the lower right edge of the board. This door is marked with the word "BONUS". In order to enter this door, enter the cannon in the lower left hand corner of the Zone: while in the cannon, press and hold right on along with and simultaneously. Hold these buttons until the cannon fires Sonic, who will be dropped on a secret platform holding the switch for the Bonus Stage's door.
When the player clear Sonic Labyrinth without getting the sixth Chaos Emerald and receive the game's bad ending, the game will present the player with a cryptic hint as to where the Bonus Stage (and the missing Emerald) may be found.
The Bonus Stage is a small level consisting of purple and magenta-checkerboard patterned square boards. It primarily consists of two large boards that are connected to each other by smaller boards. These fields sit in a bubbly black void with green support bars underneath them.
It is possible to fall over the edge of the boards; should that occur, the player will be expelled from the Bonus Stage. In addition, the Bonus Stage has a time limit of thirty seconds; once that time is up, the player will be expelled from the Bonus Stage as<a well.
Once inside, the goal of the Bonus Stage is to grab as many Rings as possible within the scant thirty second time limit. Once the player falls off the board or runs out of time, they will returned to Zone 2-3. Note that while the last Chaos Emerald is never actually seen, it is automatically added to the player's stock regardless of how many Bonus Stage Rings they have managed to nab. Clear the game with all six Emeralds to view the good ending and receive the Level Select code.