A close-up shot of a crack in a gray wall wouldn't be of much help in finding a secret. So, sometimes the screen is of the general area near the secret, and you can read the description for the exact location. Futhermore, the screens are taken of the SVGA version of the game and brightened up a tad to get a good view of the area without lighting or dark patches to cloud the intended visual. Following each area name is a map name in parentheses, i.e., the name you use to get directly to the map. Keep in mind that loading a map by typing Map mapname at the console does not always start you in the same place that you'll start if you get there by exiting an adjoining map.
Some maps have more than one entrance.
Some secrets cannot be exposed until you do things on different maps and come back. These are noted, of course. Jump in the first open sewer you come to and follow the tunnel there to its end, where the water gets deep. Go under to the left and come up in a secret room. Jump in the water by the destroyed bridge and go in the smaller opening to the left of the big round pipe.