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Aliens:
On the first level, go to the outhouse. Toss a stick of dynamite in it and blow it up. Go and look and you'll see the opening the dynamite made. Jump down it into the sewage and swim down. At the bottom is the opening to a secret area. Go through it, swim to the surface, and go down the tunnel. At the end is a strange colored cylinder. Go to the other side of it, open the little door in it, and look in to see the aliens.

Cheats:
OLIMYELLA Invinciblity
OLER Full health
OLPOSTAL All weapons and ammo
OLASH Unlimited ammo
OLWIMPY Instant reload
OLJACKPOT Adds inventory items
OLOPEC Adds oil
OLGUSHER Unlimited oil
OLAIRHEAD Fly mode
OLREDLITE Freezes enemies
OLBOUNCE Super jump mode
OLCDS Full map
OLSCREENING Show cutscenes
OLFPS Display frames per second
OLGPS Display current coordinates
OLTOMBSTONE Instant death
OLASH Unlimited ammo
OLWIMPY Instant reload
OLJACKPOT Adds inventory items
OLOPEC Adds oil
OLGUSHER Unlimited oil
OLAIRHEAD Fly mode
OLREDLITE Freezes enemies
OLBOUNCE Super jump mode
OLCDS Full map
OLSCREENING Show cutscenes
OLFPS Display frames per second
OLGPS Display current coordinates
OLTOMBSTONE Instant death

Codes:
Type these codes in during gameplay to use them.

Invincibility OLER
Full health OLPOSTAL
All weapons and ammo OLASH
Unlimited ammo OLWIMPY
Instant reload OLJACKPOT
Adds inventory items OLOPEC
Adds oil OLGUSHER
Unlimited oil OLAIRHEAD
Fly mode OLREDLITE
Freezes enemies OLBOUNCE
Super jump mode OLCDS
Full map OLSCREENING
Show cutscenes OLFPS
Display frames per second OLGPS
Display current coordinates OLTOMBSTONE
Instant death OLZIP
Teleport Levels codes: OLHIDEOUT OLTOWN OLTRAIN OLCANYON OLMILL OLSIMMS OLMINER OLCLIFF OLRANCH.

Codes:
OLPOSTAL - Full weapons / ammo
OLCDS - The whole map
OLASH - Unlimited ammo
OLREDLITE - Freeze enemies mode
OLAIRHEAD - Fly mode
OLJACKPOT - Inventory filled
OLIMYELLA - Teleport mode
OLZIP - Invisibility
OLER - Full health

Crazy Abe's Hideout:
On the second level, Sanctuary, look around town for the water tank. Jump in it and swim to the bottom. At the bottom is an opening to a secret area. In the deepest part of the cave is a guy who I think is Crazy Abe from the furniture warehouse in Sanctuary.

Level codes:
OLHIDEOUT
OLTOWN
OLTRAIN
OLCANYON
OLMILL
OLSIMMS
OLMINER
OLCLIFF
OLRANCH