Area 51 Power-up Room
After your first progress report in Area 51, where it tells you your
percentage and things like that, you will see a wall lined with
windows. Shoot all the windows across the entire wall and you will be
in the power up room, where you can see aliens attacking women and
power-ups all over the screen.

Area 51 Secret Area before Final Boss
Right before you fight the Final Boss when you are outside, you will
go to an area with zombies throwing barrels [NOT GRENADES]. Shoot all
the barrels that are not thrown, and you will go to a secret area
filled with power ups.

Area 51 Secret Level
In the office building, you will go into a series of 2 or 3 doors with
nameplates on all of them, shoot them all and then you will go into a
secret room with power ups and an alien girl at the bottom.

Area 51 Kronn Hunter Mode
To enter Khronn-hunter mode, ignore the aliens in Level 1 and shoot
the policeman (three times) with your first three bullets.

Area 51 Secret Bathroom
To see the alien restroom, you must shoot three blue lights that
appear in the corners of the screen. At the beginning of the game,
when you enter the building, shoot the first blue light in the upper
left-hand corner of the screen. At the conclusion of level 1, while
the screen is spinning, shoot the remaining two blue lights in the
upper right-hand corner of the screen.

Area 51 Hidden Level
On the forklift ride shoot all the barrels before and after the ride.
There will be a partially hidden barrel you will have to shoot to the
left of the forklift. This will lead you to "egg cellent" where you
shoot eggs and baby aliens.

Area 51 Secret Level 2
On the forklift ride shoot all the barrels before and after the ride.
There would be a partially hidden barrel you would have to shoot to
the left of the forklift. This would lead you to egg cellent where you
shoot eggs and baby aliens.

Area 51 Secret room 3
On the helicopter ride, shoot all of the boxes with flames on them. If
done correctly, you will be sent into "X Marks The Spot", which is a
room where you keep breaking the glass on a wall with your guns to
reveal a picture of a female policeperson lying there in a very
inviting position.