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It was once theorized that the moon might have deep oceans of dust, potentially a problem for any craft landing on its surface. I've heard that NASA was concerned about this possibility in the early days of the Space Race.
Is there any evidence that deep lunar dust was an actual concern at NASA?
Apparently the depth of the lunar dust has become a topic of discussion for young-Earth creationists, making it incredibly hard to search for useful information about the topic.
Yes.
The fear that lunar dust was fine, deep, and motile enough that a lander could sink under its surface was mostly backed by Thomas Gold, a noted astrophysicist who was a consultant to NASA in the 1950s.
Note especially in the image below of a Popular Mechanics article from 1964, the quote from his article in Science magazine on the subject: "Without any clear signs [in Ranger images] of firm rock, the pictures must lead to more concern about sinkage of impact."
He became in the small minority in this thinking over time and isolated in his position, but he stuck to his guns. There were some at the time who felt no more needed to be known about the lunar surface before attention could turn to sending people.
Note that this makes him the first person to recognize that the surface of the Moon has been primarily shaped by impacts. The entire surface of the Moon is covered with dust that is tens of meters deep (mixed with larger particles) entirely produced by meteor impacts pulverizing it and throwing it around over billions of years. (You don't sink in it, but it's there.) This understanding led to the realization that all solar system bodies experienced high levels of collisions for large parts of its history, and research later established that the Late Heavy Bombardment occurred, and that impacts were a major mechanism of the solar system's evolution. In turn, this realization probably influenced the development of the theory that the Moon was in fact created by an impact event between early Earth and an object about the size of Mars.
The first Surveyor mission was thus in large part meant to determine whether the dust on the surface would support a spacecraft. From Wikipedia on the Surveyor program:
Several Surveyor spacecraft had robotic shovels designed to test lunar soil mechanics. Before the Soviet Luna 9 mission (landing four months before Surveyor 1) and the Surveyor project, it was unknown how deep the dust on the Moon was. If the dust was too deep, then no astronaut could land. The Surveyor program proved that landings were possible.
The best physical evidence is the fact the foot pads of the lunar lander were dish shaped pads and not something smaller. This is not evidence of DEEP dust, but of at least the belief there was SOME dust likely to sink into.
There is an excellent series called "Moon Machines" that is often shown on the Science Channel and is also available on DVD. The episode "The Lunar Module" discusses the fact that NASA was concerned that lunar dust might be deep enough for the lunar lander to sink into it. In my opinion, this series is an excellent documentary of the Apollo Program well worth the time spent to watch it.
NASA SP-530 "Apollo Expeditions to the Moon", edited by Edgar M. Cortright, which is available on the NASA History Program Office website, states that prior to the landing of Surveyor I on the moon:
Some scientists had theorized that astronauts could be engulfed in dangerously deep dust layers
It does not detail whether these claims were taken seriously by NASA, but it was at least worthy of mention when compiling this record.
Whether covered in dust, or cheese, obviously they had to consider how to get a craft off the surface to fulfill the requirement of landing a man on the moon and "returning him safely to the earth." The last thing you want is an unsteady footing in the lander such that the return vehicle would not operate as intended.
The unorthodox wheels and 12 inch ground clearance on the lunar rover are probably the strongest unspoken evidence that they had no idea what the consistency of the surface was and needed to cover all bases. They did not even deploy the rover until missions had already touched down and returned and dust was still a concern.
