Mission 3: Fall back in the water where the skullheads
were located.
Mission 3: You will start near the Judge of Death.
Press X to get the secret mission.
Mission 3: After you get chased by the Phantom,
at the end of the hall will be a door to the left.
Enter it.
Mission 4: Return to the room with the power-up
statue. You will have an encounter with the spider
in the hall, just escape to the nearest door,
then go back out.
Mission 4, Part 2: Go to the airplane room after
the panther has been defeated, then take the elevator
down.
Mission 7: Kill the enemies in the sewer hall,
then go the pipe room (where the key was located).
Mission 11: Go through the door at the start
of the mission.
Mission 14: Check the skeleton of the boat from
the previous mission.
Mission 15: Check the skull carving underground
in the previous mission.
Mission 16: Defeat the blob Boss, then go to
the airplane room.
Mission 16: For a second secret mission, go to
the coliseum.
Mission 17: Go through the warp zone, then cross
the broken bridge without falling.
Mission 21: Check the wall on the opposite side
of the power-up statue.