D1, secret levels?

Hey guys, I am playing D1 again and having such a blast with this game, now

that I can play it at high res on XP. Thank God for D2x-xl!

Anyway, I found a secret level already on level 10 I think. That was fun 
but I had to do it so many times because I could not save and did not want

to lose my hostages. I did it by using the cloaking power ups to sneak up 
on those rotten fusion gun purple big bad boys, and then used my 
invulnerable power up to get past the reactor. How mad I was when I
finally 
got through the level and could not find my way out of the reactor room 
with all of my hostages after my invulnerability power up pooped out!

Now I am on level 17 I think and dammit, I saved the game right when I was

in a big room with a lot of bad enemies. The game locks up in there and I 
end up not being able to play. I used the auto backup but it saved the
game 
in the same spot so I am screwed. Well, I can play from the exit of the 
level before, just have to do the level over again.

So where are the three secret levels in D1? I do not want to miss any. I 
found level 10, what are the other levels where secret levels can be
found? 
Instructions as to how to find them would be bitching and a screencap
would 
be awesome. I would settle for the level and general location though. 
Thanks.

-- 
~Ohmster
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