First, I tried using the NGS Data Explorer to locate the benchmark (in Devils Golf Course) that they mentioned. I couldn't find it there. But it is shown on the 1952 Furnace Creek 15' topo map.Thank you for contacting AskUSGS.
There are a number of benchmarks throughout the valley set by the National Geodetic Survey (NGS) although apparently not one marking an actual lowest point. The run of markers were placed along the western edge of the valley.
The lowest monumented elevation we could identify using the NGS Data Explorer ( http://www.ngs.noaa.gov/NGSDataExplorer/ ) is -273.48 ft. A link to the datasheet is below.
That marker is in the Devils Golf Course, north of the Badwater Basin where the lowest point would be. It is likely that one very good reason for not locating a marker there is the salt flats would be very corrosive to any concrete and metal structure and it would probably disintegrate very quickly. Note the statement in the datasheet on the condition of that marker. Also, an absolute permanent lowest point would likely not be possible as shifting in the salt flats occur through natural processes.
NGS Datasheet H 680: http://www.ngs.noaa.gov/cgi-bin/ds_mark ... Box=GS0240
According to current elevation data, there is a slight depression, west of the Badwater parking area. Elevations determined using the spot elevation tool in The National Map Viewer ( http://viewer.nationalmap.gov/viewer/ )show -281 ft in the depression, while the majority of the basin seems to average about -278. See the attached screenshots from TNM Viewer.
If there is any official monumentation, Death Valley National Park may be responsible for placement. If you haven't already, you may want to contact them.
As the datasheet they linked shows, you're wasting your time if you go looking for that benchmark: Though it was only set in 1942, by 1949 it was already in very bad shape, and by 1983 it could not be found.
I spent a little time playing with the National Map Viewer. As they noted, it shows a depresssion out in the basin. After a lot of clicking, I confirmed that the lowest point(s) you can find using this tool is -281 feet. Below are a couple of screencaps showing what If found:
Using that Map Viewer, I navigated to the coordinates I had (and that others have used) for the -282 foot point. These coordinates come from the point shown on the 1986 Badwater 7.5' topo. Specifically, the coordinates I have are:
N 36 14 31
W 116 49 32
According to the Map Viewer, the elevation at that point (or at least, as close as I could navigate to that point in the viewer) is -279 feet:
Ultimately, their take on the situation appears to be summed up in one of the sentences in their email: "...an absolute permanent lowest point would likely not be possible as shifting in the salt flats occur through natural processes."