Animated flat

A number of floor (and ceiling) names are built-in to the Doom engine (including Heretic and Hexen) for use as animated flats. The animation gives the flat a liquid appearance. These flats are used frequently for floor textures (especially damaging floors), but rarely for ceilings. In Doom, the built-in names are:



At initialization time, the engine looks up each lump named in the "First" column above, and links it with all following lumps, up to and including that named in the "Last" column.

If any of the linked names is used as the floor (or ceiling) of a sector, then the engine changes it to the next linked name about three times a second (wrapping around to the first after using the last).

In the "Version" column above:
 * "All" refers to all versions, including the shareware
 * "R" refers to registered Doom/Ultimate Doom
 * "2" refers to Doom II

The "Frames" and "Description" columns refer to the graphics distributed with the original games. It is difficult, but possible, to replace the animated flats using a PWAD. (In particular, a PWAD must contain all the flats used, not just those replaced.) In that case, the number of frames, and of course the description, may vary.

The SWATER textures are most likely reserved for mods.

Source

 * Unofficial Doom Specs