It has a linear voxel-like appearance coming out of the floor. It'll crash if you get too close to the edge.
I also noticed music fading out and restarting as I traversed this area.
EDIT: apparently it's been part of Build since early 1994 and it is called "Groudraw" and it relied on heightmaps.

A convmap4/5/6/7 later on that world one, and in the latest Build test game... we see a slope!

Groudraws were retired for slopes.
Sector fields used for slopes:
The useless groudraw fields, ceilingheinum and floorheinum,
are now being used as the slope value. They are treated
as signed shorts, where 0 is slope 0, of course. To enable
slope mode, you must also set bit 1 of the ceilingstat or
floorstat bits.
Looks nice on floors but I only saw it working with floors.

Maybe Ken could shed some light on this groudraw stuff. I didn't see any use of it in any games prior to Duke3D since ... well, it is indeed crashy:
G = Make the floor of the given sector have a Groudraw
(floor with height mapping). I do not recommend using this
attribute very extensively yet. (See the H key for selecting
the height map)
It makes me wonder...... where could Duke Nukem 3D have used groudraw floors? Obviously not going to be places with it as the entire floor, as groudraw is slow as hell.