Whipped this up today, a retexture of a level from the original AvP99, which I call The Real Sewer. As you can probably guess, I wanted to make it look like an actual sewer. I think the original is ugly and dumb, and while it's already dark, it looks nothing like an actual sewer because it just recycled textures from the marine bases in the campaign. Installation is the same as with Fear Factory; locate your AvP Classic directory and add the folders to it, combine when prompted.

Link: http://avpunknown.com/avpclassic/files/maps/realsewer.zip

Comparison shots below: