https://www.ufopaedia.org/index.php?title=LOFTEMPS.DATthere's a lot of info here, as well as links to BB's tools (which can convert your terrain data to "visual" form)
you can also hit F10 in-game to get a POV shot of the 3d voxels, or if you have debug enabled, F11 will output the entire map as a series of vertical "slices"
the slices themselves are all pre-defined in loftemps.dat, the mcds contain an array of 12 references to this.