After destroying the caterpillar at the end of the very first stage, a little angel floats out of the body and hovers about for a few seconds. Collecting it will take you here. Basically, you fly along, collecting the angels and avoiding the demons.
Flying down the molehill after you destroy the mole takes you to this underground bonus stage. I'm not sure what the blue fireball things are supposed to be (maybe blue fireballs), but they bounce around the tunnels, and the stage is over if they hit you. You have to collect the pink crystal flowers.
Amazingly, I was completely unaware of this bonus level for over a decade. Thanks to Dr S for pointing this one out!
Before you get to the lobster at the end of this level, you have to navigate a network of stone tunnels. If you take the uppermost tunnel and keep going, it eventually leads to a dead end. However, if you fly into the wall at the end of the tunnel, it takes you to a bonus stage!
It's not too exciting if you've already figured out the molehill, though, because it's exactly the same as the Underground bonus stage on Level 1, albeit with more pondish music.
A bonus stage during a big boss battle? Indeed. As the pike descends toward you at the beginning of the battle, you can fly into its open mouth, which takes you to this aptly-named bonus stage. You fly through the pike's body, avoiding the huge jutting bones and the bouncing fishbones, collecting what appear to be... er... coke cans.
Eww. I'm still not sure if this is a bonus stage or not. It's not given a name. Anyway, if you destroy the maggots on the dead rat, you can fly into the rat's body. You end up flying through its still-heaving guts, accompanied by horrible sloshy sounds.
There doesn't seem to be anything to collect, but you have to fly through gaps in those long, green things. Don't know what they're supposed to be. If you hit one, the stage ends.
(screencap from girv.net)
The story to this is pretty neat. For years I had suspected there was a bonus stage on level 4, because there was a piece of music in the soundtrack that was part of the Techno Party! suite, but wasn't ever played in game.
It seems I wasn't the only person who had been wondering about this. On 17th February 2008, John Girvin announced that he had located the entrance to the stage by examining the game's code (not the source, the actual compiled game code!). So for the first time in 16 years, the lost Speed of Light bonus stage was seen.
It's actually not that difficult to get into, but the entrance is somewhere that you would never fly into unless you were deliberately trying to die, so it's not surprising this has never been seen before. In fact, in my flythrough of level 4, the entrance doesn't even appear on the screenshots, because I had no idea there was anything of interest there.
The stage is in fact very similar to a bonus stage in one of the Project-X games. Presumably this is a coincidence. The bee begins moving slowly through a tunnel, and gradually picks up speed, while obstacles appear in its path. If you hit one, the bonus stage ends.
John Girvin's page on his discovery of the stage also has more pictures, a video of the stage, and a patch to fix the bug for WinUAE users.Back