Secret Level:
 
To find The Underearth level you must be in The 
Crypt of Decay. When you see the exit, go straight
back. You should be in a room with boxes and a 
button on the ceiling. Shoot the button, then jump
on the boxes into the secret room. Push the button,
then go to the hidden "Well of Wishes." Now look 
around the walls and a wall will open for you.
When you get out, there will be a underwater doorway
that leads to the secret level.