The floor here is not paved, but covered in dirt, gravel, and sand. As characters walk, 1d4 of them must make a Dex or Saving Throw, or be caught in fast-moving dry quicksand. Characters have just 1-2 minutes (50-50% chance) to find some way out.
Magic might help. A long pole which extends beyond the area of the sinking sand would allow them to hold on for a while. Others outside the sand might be able to throw a rope or pole to drag the victims out, but any close approach will similarly trap potential rescuers. Characters who sink under will suffocate in 1d4 minutes.
Thieves can use their skills to detect that something is wrong with this part of the dungeon, though they aren't likely to know what that is, nor can they disarm it in the traditional sense. Characters with desert outdoor survival skills, or at the least, some desert traveling experience, may or may not be able to detect this trap in advance, at the GM's discretion.