Dry quicksand is loose sand whose bulk density is reduced by blowing air through it and which yields easily to weight or pressure.
Writing in Nature, physicist Detlef Lohse and coworkers of University of Twente in Enschede, Netherlands allowed air to flow through very fine sand (typical grain diameter was about 40 micrometers) in a container with a perforated base.
[1] Lohse found that a weighted table tennis ball (radius 2 cm, mass 133 g), when released from just above the surface of the sand, would sink to about five diameters.
The successful landings of the unmanned Surveyor probes a few years earlier and their observations of a solid, rocky surface largely discounted this possibility, however.
The large plates at the end of legs of the Apollo Lunar Module were designed to reduce this danger, but the astronauts did not encounter dry quicksand.