Its a warp I believe if your 1000 digit score is even at the end of your level you destroy 4 hatches and then you warp.

In level 3 if you destroy 10 stone heades you warp to level 5 after beating level.


* Full option command pause game UU DD LR LR BA start yes this code works alot of places even in Half Life 2


I really used to like this game.

I don't know; sometimes it seems kind of random.  I remember trying to beat the core really fast & not getting it, but by casually playing having it work.

That's what I thought too, I'm pretty definite this is a time thing.  I believe you can only do it on stages 1 and 4.  You need the laser and both options and you basically have to have them lined up when it appears and blast the crap out of it while dropping missles.  Do it fast enough and it's over.  You have to keep at least one of your lasers right in the middle of it to take it out quickly.  You'll know if you got it because the core's little barrier things will rapidly disappear.